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Chapter 17: Aids to Health

In January 1897, a doctor in Bombay rolled up his own sleeve and asked an assistant to inject him.

The doctor was Waldemar Haffkine. Plague had reached the city a few months earlier, and there was no treatment for it. Working in a small laboratory at Grant Medical College, Haffkine had grown plague bacteria in flasks of broth, killed them with heat, and prepared a liquid that contained the dead germs and nothing living at all. If dead germs could not cause the disease, he reasoned, perhaps the body would still learn to recognise them — and be ready if the living ones ever arrived.

He had done something similar once before. Five years earlier, in Paris, he had tested a cholera preparation on himself, and had then carried it to India and offered it village by village.

The plague injection gave him a fever and a painful arm for a day or two. Nothing worse followed. Over the next months, thousands of people in Bombay were offered the same preparation, and far fewer of them fell ill than those who refused it.

Haffkine had not invented a cure. He had done something stranger: he had taught a body to defend itself against an enemy it had never met.

Diagram

Fig. 17.0-1 — Preparing A Vaccine From Killed Germs Simple line drawing of a laboratory bench in three stages, left to right; label flask of broth with growing bacteria, heating of the flask to kill the bacteria, and filling of a glass syringe from the killed-germ preparation; add an arrow beneath running left to right marked ‘living germs → killed germs → injected preparation’.

NoteCuriosity Corner

Q1. How can germs that are already dead protect a person from the living ones? Q2. Long before any injection is given, what stops the millions of germs sitting on your skin from getting inside you? Q3. A newborn baby has never met a single germ. Where does its first protection come from? Q4. Why is the injection given after a snake-bite prepared in the body of a horse? Q5. A mould growing by accident in a dish of bacteria once changed medicine completely. What was it doing to the bacteria?

17.1 Need To Keep Healthy

Every one of us wants to stay healthy and free of disease. Staying healthy, however, is not something you can do entirely on your own.

Some of the work is yours alone, done with your own body and your own habits. The rest depends on the place you live in, and no single person can manage it. So the effort has to be made at two levels — the personal level and the community level.

17.1.1 Care At The Personal Level

At the personal level, you look after your own body. This means washing and bathing, brushing your teeth, taking regular physical exercise, eating a proper diet, and getting proper rest and sleep.

Each of these does a definite job. Washing removes the germs that settle on the skin from the air and from whatever you touch. Brushing removes the food left between the teeth, which bacteria would otherwise feed on. Exercise, a proper diet and enough sleep do something less visible: they keep the body’s own defences working well, so that germs which do get in are dealt with quickly.

For one day, count how many times you touch a door handle, a railing or a shared desk before you next wash your hands. You are not looking for a number to be ashamed of — only for where the chances of picking up germs are highest.

17.1.2 Care At The Community Level

No amount of personal cleanliness helps if the surroundings are dirty. At the community level, the whole neighbourhood has to be kept clean, with no garbage piling up and no stagnant water standing about.

The reason is that insects and pests which spread disease need such places to breed in. Mosquitoes lay their eggs in still water, and the young stages develop there; flies breed on rotting garbage. Clear away the water and the garbage, and the insects have nowhere to multiply — so the diseases they carry from person to person spread far less.

World Health Day is observed on 7 April every year. The date marks the founding of the World Health Organization in 1948. Each year the day is used to draw attention to one health issue and to spread the message widely.

17.2 Immunity And The Body’s Two Levels Of Defence

You are never alone. At this moment there are microbes on your hands, in the air you are breathing and on every surface around you. Most are harmless, but some can cause disease, and those we call germs — a plain word for any microbe able to make you ill.

Poisonous chemicals in the air, in water and in food — pollutants — reach the body in much the same way. Against both, the body keeps up a defence that never stops. That defence is called immunity, the body’s protection against disease.

The word ‘immunity’ comes from the Latin immunis, meaning exempt from a public duty or a tax. A person who was immune was one the tax collector could not touch. The medical sense keeps the idea exactly: a body the disease cannot touch.

17.2.1 How Germs Enter The Body

A germ can only harm you if it gets in. There are four ways in.

  1. Directly through the skin.
  2. Through the mucous membranes of the eyes, the nose, and the urinary or genital tracts.
  3. Through the food we eat or the water we drink.
  4. Through the air we breathe in.

A mucous membrane is the soft, moist lining of any passage that opens to the outside. It is not covered by tough dead skin, which is why it is easier to cross than the skin is — and also why it needs defences of its own.

Notice what these four routes have in common. Each is a place where the outside world meets the inside of the body. Wherever that meeting happens, a germ has a chance. The four entry routes are shown in Fig. 17.2-1.

Diagram

Fig. 17.2-1 — The Four Routes By Which Germs Enter The Body Outline line drawing of the human body with four labelled arrows pointing inwards; label skin, mucous membranes of the eyes, nose, urinary and genital tracts, food and water taken in through the mouth, and air breathed in through the nose.

Zoom in: a germ is a single cell, or smaller. Zoom out: the skin it must cross is an organ, and the body it threatens is a whole organism. The whole of this chapter is about one very small thing meeting one very large one.

17.2.2 The Two Levels At Which Defence Works

The body’s answer comes in two stages, and the order matters.

First, it tries to stop germs from getting in at all. Second, if some do get in, it fights them inside the tissues so that they are rendered harmless — that is, so that they can no longer cause disease. Keeping an enemy out is cheaper than fighting one already inside, so the body does the cheap thing first.

These two stages are two different systems.

A. The local defence system. This is a barrier system. It tackles germs at the points where they might enter, and it includes protective mechanical barriers; the throwing out of anything that does get in; germ-killing secretions; and germ-fighting white blood cells.

B. The immune system. This deals with germs only after they have entered the body tissues.

The whole sequence is set out in FC 17.1.

Flowchart

FC 17.1 — The Body’s Two Levels Of Defence Germ reaches the body surface → LOCAL DEFENCE SYSTEM (mechanical barriers → thrown out if entered → germ-killing secretions → germ-fighting white blood cells) → branch A: germ stopped, no disease → branch B: germ crosses into the tissues → IMMUNE SYSTEM (antibodies and antitoxins in the body fluids) → germ or its poison rendered harmless

Q. Which of the two systems is at work when the skin keeps a germ out, and which when a germ already in the blood is destroyed?

A. The skin belongs to the local defence system, which works at the entry points. Destroying a germ already inside the tissues is the work of the immune system.

17.3 The Local Defence System

The local defence system works at the entry points. It does not know or care which germ it is dealing with; it simply makes the entry points hard to cross. It works in four ways: mechanical barriers, throwing out whatever gets in, germ-killing secretions, and germ-fighting white blood cells. Fig. 17.3-1 brings them together on one body.

Diagram

Fig. 17.3-1 — The Body’s Local Defences Outline line drawing of the human head and trunk with labelled arrows to each defence; label tears washing germs from the eye, hairs in the nostrils trapping dust, mucous membranes lining the nose, mouth and lungs trapping germs, white blood cells circulating in the blood, acid in the stomach killing swallowed germs, and skin as the outer barrier.

17.3.1 Protective Mechanical Barriers

These are the physical obstacles a germ meets before it meets anything else — the skin, the hair and the mucus.

17.3.1.1 Skin

The skin has an outer layer made of a tough protein called keratin, and this layer is almost impermeable to germs — that is, germs cannot pass through it.

At any moment there are large numbers of germs sitting on the outer surface of your skin. They arrive through the air, and through direct contact with contaminated objects and with other people; even a handshake passes germs from one person to another. Sitting there, they do no harm. Washing with soap and water removes them.

The weak point is a break in the surface. Any scratch or cut opens a way in. The body closes that way quickly: blood clots at the wound and the clot plugs the cut ends of the blood vessels, so the opening is sealed before many germs can use it.

Skin Outer layer of dead cells filled with tough keratin → almost impermeable and continuous over the whole body → germs settling on it cannot cross, and are simply washed away.

Q2. Long before any injection is given, what stops the millions of germs sitting on your skin from getting inside you?

A. The keratin layer of the skin. It is tough and almost impermeable, so germs resting on the surface have no way through — unless a scratch or cut breaks it, and a blood clot then seals that break.

17.3.1.2 Hair

Hair on the body surface intercepts a germ’s journey inwards before it reaches the skin. The hairs inside the nostrils do the same job at the entrance to the air passage, trapping dust particles — and the germs riding on them — from the air you breathe in.

17.3.1.3 Mucus

Mucus is the slimy secretion made by the epithelial lining of many organs. Being sticky, it holds on to whatever lands in it.

The mucus in the nasal passage and the windpipe traps large numbers of bacteria and keeps them from reaching the tissues. Trapping alone would not be enough, though, because the trapped germs would collect there. The lining of the windpipe also carries cilia, tiny hair-like structures that beat steadily and sweep the mucus, with its load of bacteria, back up and out of the air passage.

Mucus and mucous. Mucus is the slimy secretion itself. Mucous is the describing word, used for the lining that makes it — a mucous membrane secretes mucus.

17.3.2 Thrown Out, If Entered

Some germs and foreign particles get past the barriers into the respiratory and digestive passages. The body then throws them out bodily.

Coughing and sneezing drive them out of the respiratory passage in a fast blast of air. Vomiting empties the stomach. Even diarrhoea, or loose motions, helps flush germs out when an infection persists in the digestive tract.

Rehydration During Diarrhoea Diarrhoea clears germs out of the digestive tract, but it also carries away a great deal of water and dissolved salts, and losing those is what makes a person seriously ill. This is why a person with diarrhoea is given oral rehydration solution — clean water with a measured amount of salt and sugar in it. The solution does not kill the germ. It replaces what the body is losing while the body does the clearing.

17.3.3 Germ-Killing Secretions

Some of the fluids the body pours onto its own surfaces do more than wash — they kill.

  1. Saliva, sweat, tears and nasal secretions all contain germ-killing substances.
  2. Hydrochloric acid secreted by the stomach kills germs that are swallowed along with food.

The stomach is worth a second look. Food and water are two of the four entry routes, and everything taken in by mouth passes through a bath of strong acid before it reaches the intestine. Most swallowed germs do not survive it.

17.3.4 Germ-Fighting White Blood Cells

Should a microbe get through in spite of all this, the white blood cells are ready. Certain white blood cells are phagocytes — cells that eat other cells.

Blood travels in closed vessels, so a phagocyte in a capillary is not yet where the germ is. It gets out by diapedesis: the cell squeezes itself through the wall of the blood capillary and into the surrounding tissue. There it moves towards the bacteria and engulfs them, flowing around each one until it is enclosed and then destroying it inside itself. This engulfing and destroying is called phagocytosis, and the steps are shown in FC 17.2 and drawn in Fig. 17.3-2.

Flowchart

FC 17.2 — Phagocytosis Bacteria enter the tissue → white blood cell is attracted towards them → cell squeezes through the capillary wall (diapedesis) → cell moves up to the bacteria → cell flows around and engulfs them (phagocytosis) → bacteria are digested and destroyed inside the cell

Diagram

Fig. 17.3-2 — A White Blood Cell Engulfing Bacteria Three-part line drawing labelled A, B and C; A shows the white cell forcing its way across the capillary wall, B shows the cell moving towards a cluster of bacteria outside the capillary, C shows the bacteria enclosed inside the cell; label capillary wall, bacteria, white blood cell and its lobed nucleus.

Pus in a wound is the visible result of this fight. It is a mixture of destroyed germs, white blood cells killed in the struggle, and damaged tissue cells.

Look closely at a small healing scab on your own arm or knee — the ordinary kind everyone gets. That hard plug is the clot described earlier, still sealing the break in the skin. Notice how long it stays before the skin under it is whole again.

17.3.5 Merits Of The Local Defence System

These defences are simple, but they have three real advantages.

  1. They start working instantaneously. Nothing has to be prepared first — the skin, the acid and the phagocytes are already in place.
  2. They are not dependent on previous exposure to infection. The body does not need to have met the germ before.
  3. They are effective against a wide range of potentially infectious agents, because they do not single out any one kind of germ.

That last point is a strength and a weakness at the same time. A defence that treats every germ alike cannot be especially good against any one of them — which is exactly the gap the immune system fills.

17.4 The Immune System

The local defences are good, but they are not enough in every case. Some microbes have special ways of getting in, and any breach in the barriers — a deep cut, a needle, an insect bite — lets others through. Certain germs also send in their poisonous secretions, called toxins, ahead of themselves.

So germs and toxins do reach the deeper tissues and the organ systems. Once they are that far inside, no barrier can help, and the fight has to be carried out in the body fluids themselves.

17.4.1 Antibodies And Antitoxins In The Body Fluids

At this stage the blood and the other body fluids begin fighting anything that does not belong to the body. They do it with special proteins.

Antibodies are proteins in the body fluids that react with the invading germs themselves. Antitoxins are proteins that react with the poisons those germs release, and destroy them. Between them, the two deal with both parts of the threat — the germ and what it produces — and this is how the body fluids protect against the disease.

Antibody and antitoxin. Both are proteins made by the body against something foreign. An antibody acts on the germ itself; an antitoxin acts on the poison the germ has released.

17.4.2 What Immunity Means

You now have the full picture, and the word ‘immunity’ can be given its proper meaning. It does not mean that germs never reach you. It means that when they do, the body resists them, so that the disease does not take hold.

NoteDefinition

Immunity The resistance of the body to the onset of a disease after infection by harmful germs.

Put more fully, immunity is the capacity of the body to deal with foreign substances — bacteria, viruses, toxins and the like — that enter it, and to render them harmless.

Q. A germ enters a cut and multiplies in the tissue, but the person never falls ill. Has the body’s immunity worked, or failed?

A. It has worked. Immunity is not the absence of germs but resistance to the disease; the germ entered and was rendered harmless.

When The Immune System Itself Fails AIDS — acquired immuno deficiency syndrome — is caused by infection with HIV, the human immunodeficiency virus. It spreads through sexual contact, through contaminated syringe needles and through transfusion of infected blood. What makes it serious is where the virus attacks: it destroys cells of the immune system itself, greatly reducing the T-cells that switch on the lymphocytes producing immunity. As immunity collapses, other infections that the body would normally have handled develop unchecked. There is at present no cure, but medicines are available that hold the virus in check for many years, and a person taking them can stay well and cannot pass the virus on through sexual contact. AIDS is dealt with in full in an earlier chapter of this book.

17.5 Kinds Of Immunity

Not all immunity comes from the same place. Some of it you were born with; some of it you have collected since. That single difference splits immunity into two main kinds — innate immunity and acquired immunity — each of which divides again, as set out in FC 17.3.

Flowchart

FC 17.3 — The Kinds Of Immunity IMMUNITY → branch 1: INNATE (inborn, natural) → (a) non-specific, general defence mechanisms; (b) genetically determined resistance, of a species, population or individual. IMMUNITY → branch 2: ACQUIRED (during lifetime) → (a) active, produced by one’s own body → (i) natural, by previous infection; (ii) artificial, by vaccination. ACQUIRED → (b) passive, supplied from outside sources → (i) natural, antibodies received from the mother through the placenta or milk; (ii) artificial, ready-made antibodies produced in other animals

17.5.1 Innate Immunity

Innate immunity is also called natural or native immunity. It comes from your genetic constitutional make-up — the inherited design of your body — and it is present without any previous infection and without anything being done to bring it about. A newborn who has never met a germ already has it.

17.5.1.1 Non-Specific Innate Immunity

This is a degree of natural resistance to all infections in general. It is called non-specific because it is not aimed at any one germ.

You have already met it in action. The skin prevents the entry of germs, whichever germs they are, and the white blood cells destroy invading microbes regardless of type.

17.5.1.2 Genetically Determined Resistance

This is natural resistance to one particular kind of germ only, and it too is inherited rather than earned.

The clearest examples are between species. Distemper is a highly infectious disease of dogs which kills about half the dogs that catch it, and human beings are completely immune to it — not because we have met it, but because our bodies are not the kind of body the germ can live in. Within a species too, the inherited make-up of some individuals or populations leaves them resistant to a particular infectious disease that affects others.

17.5.2 Acquired Immunity

Acquired immunity is resistance to a disease which an individual acquires during his or her lifetime. It was not there at birth. It may come about in either of two ways: from a previous infection, or from ready-made antibodies supplied from outside.

17.5.2.1 Actively Acquired Immunity

This is the resistance an individual develops because of a previous infection, or because of an antigen — a chemical found on the surface of a disease-causing germ — that has entered the body. Here the body does the work itself, which is why it is called active.

The antigen may arrive in either of two ways.

  • If it enters the body naturally, in the course of an infection, the result is naturally acquired active immunity. A person who has once had measles will not normally suffer from it again.
  • If it is introduced deliberately, as in vaccination, the result is artificially acquired active immunity.

In either case the same cells respond. Lymphocytes, a type of white blood cell, react in two ways.

  • They produce antibodies, which circulate freely in the blood and lymph and bind to the microorganism to kill it.
  • They produce killer cells, which carry specific receptors for the foreign antigens found on the invading germs.

Actively acquired immunity takes time to appear, because the antibodies have to be made. What it gives in return is that it is usually long-lasting, carried by ‘memory’ lymphocytes that keep the record of that antigen ready for the next encounter.

17.5.2.2 Passively Acquired Immunity

Here the individual’s own body does not produce the protection at all. It is supplied from an outside source in the form of ready-made antibodies. This too is of two types.

Naturally acquired passive immunity. The mother’s ready-made antibodies pass to the developing foetus through the placenta, and reach the baby again after birth through her milk. The baby has made none of them.

Q3. A newborn baby has never met a single germ. Where does its first protection come from?

A. From two places. Its innate immunity — skin, secretions, white blood cells — is inborn. On top of that it carries ready-made antibodies received from its mother through the placenta before birth and through her milk after it.

The first milk a mother produces after birth is called colostrum. It is thicker and yellower than ordinary milk and is especially rich in antibodies, which is why the first few feeds matter out of proportion to their small quantity.

Artificially acquired passive immunity. Here the antibodies are produced in the blood of a horse or some other animal, by injecting the germs into that animal in a mild, controlled way. The animal’s body makes antibodies against them. Its blood is then drawn, and the serum — the clear fluid part of blood, containing the antibodies — is separated out. An injection of this fluid is called an antiserum, and the whole treatment is called passive immunisation. The steps are drawn in Fig. 17.5-1.

Diagram

Fig. 17.5-1 — How An Antiserum Is Prepared Four-stage line drawing left to right; stage 1, germs or a toxin injected into a horse; stage 2, antibodies forming in the horse’s blood; stage 3, blood drawn and the serum separated in a tube; stage 4, the serum injected into a patient. Label germs or toxin injected, antibodies in horse’s blood, serum containing ready-made antibodies, and patient receiving antiserum.

This is used when a person needs protection at once and has no time to make it. A person bitten by a poisonous snake is given an injection of antivenin, which contains antibodies against the venom that were produced in the body of a horse. A person with diphtheria is given anti-diphtheria injections on the same principle. In India, such anti-sera are prepared at the Haffkine Institute in Mumbai — named after the scientist you met at the start of this chapter — and at the Central Research Institute at Kasauli.

Q4. Why is the injection given after a snake-bite prepared in the body of a horse?

A. Because a person bitten by a snake has no time to make antibodies. A horse, injected with the venom beforehand under controlled conditions, has already made them; its serum supplies them ready-made, and they act immediately.

Antigen and antibody. An antigen is foreign — a chemical on the surface of the germ. An antibody is the body’s own protein, made in answer to that antigen. The antigen comes in; the antibody goes after it.

17.5.3 Active Immunity Compared With Passive Immunity

The two acquired immunities differ in who makes the antibodies, and almost everything else follows from that. Table 17.1 sets the differences side by side.

Table 17.1 — Active Immunity Compared With Passive Immunity | Feature | Active immunity | Passive immunity | |—|—|—| | Source of antibodies | Produced by one’s own body | Received from outside; not developed by one’s own body | | How it is brought about | By infection, or by contact with immunity-producing agents such as vaccines | By supplying ready-made antibodies | | Quality of protection | Effective and long-lasting | Less effective and temporary; does not ensure protection against a later infection | | Time taken to act | Effective only after a time gap, needed for the antibodies to be produced | Immediately effective |

Active — you act; your own body makes the antibodies, so it takes time but lasts. Passive — you receive; the antibodies arrive ready-made, so they act at once but do not last.

17.6 Antibodies

Antibodies have been mentioned several times already. They are the special chemicals found in the blood which act against the germs or against the secretions those germs release. It is worth looking at them closely, because almost everything else in this chapter depends on how they behave.

What Antibodies Are

Antibodies are proteins. They belong to a class of proteins called the immunoglobulins.

They are produced by a particular type of specialised lymphocyte, and only on exposure to an antigen. No antigen, no antibody. These lymphocytes are concentrated in the lymph nodes and the spleen, and are also carried in the circulating blood and lymph, so that wherever an antigen turns up there are lymphocytes nearby.

A lymph node is a small swelling on the lymph vessels, packed with lymphocytes. When you have a throat infection, the tender lumps you can feel under the jaw are lymph nodes at work — crowded with dividing lymphocytes.

How Antibodies Act

Our body can make an unlimited variety of different antibodies. That variety is necessary, because antibodies are specific: one kind of antibody acts against one particular type of antigen only, and against no other.

An antibody recognises its own antigen and binds to it. Binding renders the antigen harmless, and the marked germ is then destroyed and eliminated by the body. Fig. 17.6-1 shows what this looks like, and FC 17.4 sets out the order of events.

Diagram

Fig. 17.6-1 — Antibodies Attack Bacteria Line drawing in three parts; on the left a lymphocyte recognising rod-shaped bacteria as foreign and secreting antibodies; in the middle antibodies binding the bacteria so that they clump together; on the right a phagocytic white blood cell taking in the clumped bacteria. Label lymphocyte, bacteria, antibodies, clumped bacteria and phagocytic white cell.

Flowchart

FC 17.4 — What Happens When An Antibody Meets Its Antigen Germ carrying its antigen enters the body → lymphocyte recognises the antigen as foreign → lymphocyte secretes the matching antibody → antibody binds the antigen and the germs clump together → phagocytic white cell engulfs the clump → germs digested and eliminated from the body

Antibody Binding site shaped to fit one antigen and no other → the antibody attaches only to the germ it was made against → healthy body cells, which carry no such antigen, are left untouched.

Two Further Points

Some people carry certain antibodies in their blood from the very birth. Such people do not suffer from those particular diseases even if the germs have crossed the barriers and escaped the phagocytes; the antibodies are waiting.

The protection antibodies give does not last the same length of time in every disease. Against some, such as the common cold and cholera, it lasts only a short period. Against others, such as smallpox and measles, it lasts far longer — which is why a person who has had measles once will normally not have it again.

Q. Why does one attack of measles protect a person for years, while a cold caught in January is no protection against a cold in March?

A. Because antibodies are specific. The measles antigen stays the same, so the same antibodies still fit it. Colds are caused by many different viruses with different antigens, and the antibodies against one do not fit another.

17.7 Vaccination And Immunisation

Actively acquired immunity is the good kind — specific and long-lasting — but the natural way of getting it is to catch the disease first. That is a high price. Vaccination is the way round it.

Vaccination is the practice of artificially introducing the germs, or a germ substance, into the body in order to develop resistance to a particular disease. Scientifically the practice is called prophylaxis, meaning protection given in advance, and the material introduced into the body is called the vaccine.

NoteDefinition

Vaccination The introduction of any kind of dead or weakened germs into the body of a living being, to develop immunity against the respective disease or diseases.

NoteDefinition

Immunisation Developing resistance to disease-producing germs or their toxins, by introducing killed germs or germ substances so as to induce the production of specific antibodies.

Vaccination and immunisation. Vaccination is the act — giving the vaccine. Immunisation is the result — the resistance the body then develops. You are vaccinated on a Tuesday; you are immunised some weeks later.

The terms ‘vaccine’ and ‘vaccination’ were originally used only for vaccination against smallpox. They are now used in a general sense for all such protection.

17.7.1 What A Vaccine Does Inside The Body

The vaccine is usually introduced by injection, and sometimes by mouth — polio vaccine is given as oral drops.

Once inside, the vaccine does not fight anything. It carries the antigen of the disease without the power to cause the disease, and that antigen stimulates the white blood cells to produce antibodies against the germs of that particular disease. The antibodies are made, and the memory lymphocytes keep the pattern. If the real germ arrives years later, the body is not meeting it for the first time. FC 17.5 follows the whole sequence.

Flowchart

FC 17.5 — How A Vaccine Protects Vaccine containing the antigen is introduced → antigen cannot cause the disease → lymphocytes recognise it as foreign → antibodies produced against that antigen → memory lymphocytes retain the pattern → real germ enters months or years later → antibodies produced rapidly and in quantity → germ destroyed before the disease develops

Q1. How can germs that are already dead protect a person from the living ones?

A. Because what the body responds to is the antigen on the germ’s surface, not the germ’s ability to make you ill. A killed germ still carries its antigen. The lymphocytes make antibodies against it and remember the pattern, so the living germ, when it comes, meets a body that is already prepared.

The word ‘vaccine’ comes from the Latin vacca, a cow. The first such protection ever used was made from cowpox, a mild disease caught from cattle, and the name stayed on every vaccine made since.

17.7.2 Four Categories Of Vaccines

Vaccines differ in what exactly is put into the body. There are four categories.

  1. Killed germs. The germs are killed first, so they cannot multiply, but their antigens survive. The TAB vaccine for typhoid, Salk’s vaccine for poliomyelitis, and the vaccine for rabies, given after a dog-bite, are of this kind.
  2. Living weakened germs. The germs are alive but have been made too weak to cause the disease. The vaccine for measles is of this kind, and so is the freeze-dried BCG vaccine for tuberculosis. BCG stands for Bacillus Calmette–Guérin, named after the two French workers who developed the strain used for vaccination.
  3. Living germs that are fully active, but of a related and milder disease. Vaccination against smallpox worked this way: a person was inoculated with cowpox virus, which is very similar to the smallpox virus. Cowpox produces only a single pustule instead of the multiple pustules of smallpox all over the body, and protection against cowpox protects against smallpox as well. Smallpox vaccination is no longer given to anyone, because the disease has been completely eradicated.
  4. Toxoids. Some bacteria harm the body through the toxins they secrete rather than by their own numbers. A toxoid is such a toxin inactivated by the addition of dilute formalin: it is no longer poisonous, yet it keeps the capacity to make the body produce antibodies — in this case antitoxins. The vaccines against diphtheria and tetanus are toxoids.

Work is still going on to develop a vaccine against AIDS.

Q. Why is a toxoid, and not the killed bacterium, used against tetanus?

A. Because in tetanus the damage is done by the toxin, not by the bacterium itself. Antibodies against the toxin are what the body needs, so it is the toxin — made harmless — that is given.

17.7.3 The National Immunisation Schedule

A vaccine protects only the person who receives it. To protect a population, the same vaccines have to reach nearly everybody, at the right ages, in the right order. India has done this on a mass scale, most recently for COVID-19 as well, with the aim of covering the entire population.

The ages at which the main vaccines are given are set out in Table 17.2. Several doses appear more than once: a repeat given after the first course, to lift protection that has begun to fade, is called a booster dose.

Table 17.2 — The National Immunisation Schedule | Age | Vaccinations | |—|—| | 3–12 months | DPT — 3 doses at intervals of 4–6 weeks; Polio (oral) — 3 doses at intervals of 4–6 weeks; BCG (intradermal) | | 9–15 months | Measles vaccine — one dose | | 18–24 months | DPT — booster dose; Polio (oral) — booster dose | | 5–6 years | DT — booster dose; Typhoid (TAB) vaccine — 2 doses at an interval of 1–2 months | | 10 years | Tetanus toxoid — booster dose; Typhoid vaccine — booster dose | | 16 years | Tetanus toxoid — booster dose; Typhoid vaccine — booster dose | | Mothers, during pregnancy | If immunised previously: one booster dose of tetanus toxoid, preferably 4 weeks before the expected date of delivery. If not immunised previously: two doses of tetanus toxoid, the first between 16 and 24 weeks and the second between 24 and 32 weeks of pregnancy | | COVID-19 vaccination | Two doses at an interval of 1–2.5 months, with a booster dose for healthcare workers, people above 60 years and people with other health conditions |

The short forms used in the table are worth knowing. DPT is diphtheria, pertussis — whooping cough — and tetanus. DT is diphtheria and tetanus. BCG is Bacillus Calmette–Guérin. COVID-19 is coronavirus disease 2019.

A schedule of this kind is revised from time to time as new vaccines become available and as diseases change in importance, and the version in force at any moment is the one issued by the Ministry of Health and Family Welfare.

Why Polio Vaccine Is Given As Drops Poliomyelitis is a disease of the nervous system that can leave a child with permanent paralysis of the limbs. Protecting every child meant reaching villages far from any clinic, and an injection needs a trained hand, a sterile needle and a cold store. Oral drops need none of these; a volunteer can give them from a vial on a table, and a child too young to sit still can still be vaccinated. That is how national polio immunisation days were able to reach children in their millions on a single day.

17.8 Toxins And Antitoxins

Not every attack on the body is made by the germ itself. Some of the damage is chemical, and the words used for it are worth getting straight.

The Term Toxin

A toxin is any poisonous substance produced by a living thing — an animal, a plant or a bacterium. The word is general, and covers a wide range of poisons.

Snake venom is a toxin. So are the sting poisons of scorpions and of insects, and so are the poisonous chemicals released by pathogens growing inside the body. What they have in common is not their chemistry but their origin: each is made by something alive.

The Term Antitoxin

Antitoxin was the name given to any chemical substance produced inside the body in response to the entry of a foreign poisonous substance. Against snake venom, such substances — the antivenins — are produced inside animals such as horses.

The name has since been overtaken. An antitoxin turned out to be the same sort of thing as any other antibody: a blood serum protein produced in response to an antigen that has entered the body. So the more general term ‘antibody’ is now used in place of ‘antitoxin’. You will still meet the older word, particularly for tetanus and diphtheria, and it means an antibody directed against a toxin.

Toxin and toxoid. A toxin is the poison as the bacterium makes it. A toxoid is that same poison treated with dilute formalin so that it can no longer poison, but can still make the body produce antibodies.

Using Ready-Made Antibodies As Treatment

Suppose a person has actually developed a disease such as diphtheria. Waiting for their own antibodies would take too long. In such a case, pre-prepared antibodies from an outside source are injected instead.

For this, an antibody-containing serum is obtained from the blood of horses, rabbits or similar animals, in which the disease has first been produced artificially in a mild form. Treatment with such antibodies is called passive immunisation, and it works on exactly the principle set out earlier for antivenin: protection borrowed, not built.

Two Injections After A Deep Wound Tetanus bacteria live in soil and enter through deep or dirty wounds, and the harm they do is done by their toxin. A person who comes to a hospital with such a wound and has not been vaccinated may be given two different injections at once. One is the tetanus toxoid, which sets the body making its own antitoxin — slow, but lasting. The other is a ready-made antitoxin, which acts immediately but fades in a few weeks. The first is active immunisation, the second passive, and together they cover both the coming days and the coming years.

17.9 Antiseptics And Disinfectants

Everything so far has been done by the body. There are also chemicals we use from outside to kill germs before they ever get a chance — some gentle enough to put on a person, others far too strong for that. The difference between the two is worth being exact about.

17.9.1 Antiseptics

Antiseptics are mild chemical substances which kill germs when applied on the body. They are used in such mild concentration that they cause no harm to the skin and to the body — which is the whole point of them, since a chemical that killed germs and skin equally would be useless on a wound.

Lysol, carbolic acid, iodine, benzoic acid, mercurochrome and boric acid, in dilute solution, are good antiseptics. Certain antibiotic creams serve the same purpose.

Dettol, Savlon, Listerine and Soframycin are commercial names, not the names of antiseptics. Each bottle contains one or more active ingredients, alone or in combination, and it is those ingredients that actually kill the germs. The name on the label is the maker’s; the antiseptic is inside it.

Lister And Carbolic Acid In the 1860s, surgery was survivable but its aftermath often was not: wounds that had been closed neatly went on to fester, and many patients died of the infection rather than the operation. Joseph Lister, a surgeon in Glasgow, had read that carbolic acid was used to treat foul-smelling sewage, and reasoned that whatever it was destroying there might be what was spoiling his wounds. He began dressing wounds with carbolic acid and treating his instruments and his hands with it. The rate of death from wound infection in his ward fell sharply. Lister had not identified any particular germ; he had shown that keeping germs out of a wound was what mattered — the principle every antiseptic still works on.

17.9.2 Disinfectants

Disinfectants are strong chemical substances applied on spots and places where germs thrive and multiply — drains, floors, refuse pits, sick-rooms. They are not applied to people.

Commonly used disinfectants are cresol, phenol, lysol, 40 per cent formalin, lime, bordeaux mixture and DDT. Of these, DDT acts on the insects that carry disease from place to place rather than on germs themselves, which is why it appears in this list of things used to clean up a site.

As a precaution, remember that all disinfectants are strong and should not come in contact with human skin.

Germs can also be destroyed without any chemical at all. Strong heat and boiling kill them just as effectively, and may be called physical disinfectants — which is why surgical instruments and, at home, a baby’s feeding bottles are boiled.

You may have noticed lysol in both lists. That is not a mistake: what decides the matter is concentration. A substance strong enough to be a disinfectant on a floor may, diluted far down, be mild enough to use on the skin.

Find a bottle of antiseptic liquid and a bottle of floor cleaner at home and read the small print on each. Look for the phrase ‘active ingredient’ and write down what each one contains, and in what percentage. Compare the two percentages before you put either bottle back.

17.9.3 Antiseptics Compared With Disinfectants

Both kill germs. Everything else about them differs, as Table 17.3 shows.

Table 17.3 — Antiseptics Compared With Disinfectants | Feature | Antiseptics | Disinfectants | |—|—|—| | Strength | Mild germ-killing chemicals | Strong germ-killing substances | | Effect on the body | Cause no harm to the skin and body | May cause harm to the skin and body | | Where applied | On the body | On spots and places | | Example | Iodine | Phenol |

One more product needs to be kept out of both columns. Deodorants are neither antiseptics nor disinfectants. They kill nothing at all; they only serve to mask a bad smell.

Q. A hospital floor is washed with phenol solution and a patient’s cut is dressed with dilute iodine. Why are the two chemicals not exchanged?

A. Phenol is strong enough to damage skin and tissue, so it cannot go on a person. Dilute iodine is mild, and would be too weak to clear the germs multiplying over a whole floor.

17.10 Antibiotics

Antiseptics and disinfectants kill germs on a surface. Neither is any use once the germs are multiplying inside the tissues, because a chemical strong enough to kill them there would kill the patient too. What was needed was something that could tell the difference — and it was found in an unlikely place. Microorganisms had been competing with one another for far longer than we have been treating disease, and some of them make chemicals that destroy their rivals.

NoteDefinition

Antibiotics Chemical substances produced by some microorganisms, that can kill or inhibit the growth of other microorganisms.

The Term Antibiotics

The first antibiotic, penicillin, was discovered in 1929, but its first use for treating a human disease was not tried until the 1940s. The name for this whole category of drugs came later still: the term ‘antibiotics’ was coined by Selman Waksman in 1942.

17.10.1 Discovery Of Penicillin

Alexander Fleming (1881–1955), working in a hospital in London, was growing laboratory cultures of Staphylococcus, a bacterium whose round cells grow in clusters.

In 1928 he found that an unwanted mould, Penicillium, had grown in one of the cultures in a petri dish. Around the mould, the bacteria had not grown at all. There was a clear space, and beyond that clear space the bacteria continued as usual — so whatever stopped them had spread outwards from the mould for some distance and no further.

That is the whole of the observation, and the conclusion follows from it. Some substance must have spread from the mould and killed the bacteria as far as it reached. In 1929 Fleming named that bacteria-killing substance released by the mould penicillin. Fig. 17.10-1 shows what he saw.

Diagram

Fig. 17.10-1 — Fleming’s Petri Dish Surface view of a circular petri dish; show a round mould colony near one edge, a clear zone with no bacterial growth in a ring around it, and dense bacterial colonies covering the rest of the dish. Label mould colony, clear zone of no bacterial growth, and bacterial colonies.

Q5. A mould growing by accident in a dish of bacteria once changed medicine completely. What was it doing to the bacteria?

A. It was releasing a chemical of its own — penicillin — which spread outwards through the jelly and stopped the bacteria growing wherever it reached. That is why a clear ring appeared around the mould and nowhere else.

In the expectation that this substance might become an effective killer of disease-causing bacteria, it was tried on laboratory animals. It gave positive results without harming the animals — which mattered, because a substance that killed bacteria and their host equally would have been of no use.

On human beings, penicillin was first tried in the early 1940s and was found to be quite effective against several infections. In particular it gave very good results in treating gonorrhoea, a sexually transmitted disease. In those early days physicians sometimes recommended a combination of penicillin with sulpha drugs, sold under the name pentidsulph. After penicillin came a whole line of further antibiotics — streptomycin, chloromycetin, aureomycin, ampicillin and many others — and these are used effectively against many infectious diseases today. Antibiotics are also used to prevent infection after surgical operations.

17.10.2 Sources Of Antibiotics

The mould that produced penicillin in Fleming’s findings was identified as Penicillium notatum. It is somewhat similar to the mould that often grows on oranges, lemons and other citrus fruits.

Commercial production of penicillin has largely been carried out from a related species, Penicillium chrysogenum, which yields far more of it, though penicillin is also produced synthetically.

Streptomycin, one of the very widely used antibiotics, comes from a different kind of organism altogether — a soil bacterium of the genus Streptomyces. So antibiotics are obtained both from fungi and from bacteria.

A large number of antibiotics are now made synthetically in factories rather than by culturing microorganisms.

Look at a slice of orange left too long in a warm kitchen and you will often find a blue-green powdery mould on it. That is a Penicillium, a close relative of the one in Fleming’s dish. The genus is common on stale citrus fruit and bread, which is partly why a stray spore reached his petri dish at all.

17.10.3 How Antibiotics Act On Germs

An antibiotic does not attack a bacterium at random. It interferes with one particular thing the bacterium must do in order to live, and it chooses something our own cells do not do in the same way. Two examples show this.

  • Penicillin interferes with the formation of the bacterial cell wall. Without a proper wall the bacteria are unable to grow and multiply. Our own cells have no cell wall at all, which is why penicillin can be given to a patient safely.
  • Streptomycin binds to bacterial ribosomes, the structures on which proteins are built. Bacteria treated with it are unable to synthesise their proteins, and so are unable to grow and multiply.

Antibiotic and antiseptic. An antibiotic is made by a microorganism and works against germs inside the body. An antiseptic is a chemical applied on the body surface. One is swallowed or injected; the other is dabbed on.

17.10.4 Uses Of Antibiotics

  1. Primarily, antibiotics find wide use in medicine to fight infections.
  2. Certain antibiotics are used as food preservatives, especially for fresh meat and fish.
  3. Some are used in treating animal feed, to prevent internal infections in livestock.
  4. Some antibiotics are used for controlling plant pathogens.

Why A Course Of Antibiotics Is Finished In any large population of bacteria, a few happen to be less affected by a particular antibiotic than the rest. A full course kills the sensitive ones and then, over the remaining days, the stubborn ones as well. If the medicine is stopped as soon as the fever settles, it is exactly those stubborn bacteria that survive and multiply, and the next infection they cause will not respond to that antibiotic. This is why a course is completed as prescribed, and why antibiotics are of no use at all against a common cold — a cold is caused by a virus, which has neither a bacterial cell wall nor bacterial ribosomes for the drug to act on.

17.11 Sulphonamide Group Of Medicines

Antibiotics were borrowed from living things. There is an older line of attack, in which the medicine is not borrowed from anything — it is built.

Since ancient times people have searched for medicines to cure disease. Many plant and animal products were tried, and a good number of them worked. Various chemical substances were tried too, and some of these also yielded good results. Out of that long search grew chemotherapy — the treatment of disease by the use of chemicals.

Salvarsan

In 1910 a drug called salvarsan, based on an arsenic compound, was produced. It killed the germs of syphilis, and was used against sleeping sickness as well.

Salvarsan was a genuine step forward, and it also showed exactly where the difficulty lay. Frequently it proved poisonous to the patient too. A chemical that kills germs is easy to find; a chemical that kills germs while leaving the person unharmed is the hard part, and every drug in this chapter has had to solve it.

Paul Ehrlich, whose laboratory produced salvarsan, described what he was hunting for as a ‘magic bullet’ — a chemical that would travel through the body and strike only the germ, passing every one of the patient’s own cells by. The phrase has stayed in medicine ever since.

The Sulpha Drugs

In the 1930s a group of chemicals known as the sulphonamides was discovered, and these proved effective in many types of bacterial disease. Sulphadiazine and sulphanilamide are two such sulpha drugs.

They are synthetic drugs — made in a laboratory, not obtained from any microorganism. They act by interfering with the metabolism of the bacteria, that is, with the chemical reactions the bacteria must carry out to stay alive, and the bacteria are killed as a result.

Sulphonamides are now rarely used, and even then mostly in combination with antibiotics, for certain ailments.

Antibiotic and sulpha drug. Both kill bacteria inside the body. An antibiotic was originally obtained from a microorganism; a sulpha drug was never obtained from anything living — it was synthesised from the start.

Taken together, the local barriers, the immune system, vaccination, the antiseptics and the drugs are not five separate topics. They are five points along the same path a germ has to travel — and it has to get past every one of them.

17.12 Summary

  1. Staying healthy needs care at the personal level — washing, bathing, brushing the teeth, exercise, proper diet, rest and sleep — and at the community level, where clean surroundings with no garbage or stagnant water stop disease-spreading insects and pests from breeding.
  2. Immunity is the body’s defence against disease. Germs and pollutants enter the body by four routes: directly through the skin, through the mucous membranes of the eyes, nose and urinary or genital tracts, through food and water, and through the air breathed in.
  3. The defence works at two levels: the local defence system, which prevents entry, and the immune system, which deals with germs after they have entered the tissues.
  4. The local defence system works in four ways — protective mechanical barriers, throwing out whatever enters, germ-killing secretions, and germ-fighting white blood cells.
  5. The mechanical barriers are the skin, whose outer keratin layer is almost impermeable and whose breaks are sealed by blood clots; hair, which intercepts germs and traps dust in the nostrils; and mucus, which traps bacteria and is swept out of the windpipe by cilia.
  6. Coughing, sneezing, vomiting and diarrhoea throw out germs that have entered the respiratory and digestive passages.
  7. Saliva, sweat, tears and nasal secretions contain germ-killing substances, and hydrochloric acid in the stomach kills germs swallowed with food.
  8. White blood cells that are phagocytes leave the capillaries by diapedesis and destroy germs by phagocytosis. Pus is a mixture of destroyed germs, killed white blood cells and damaged tissue cells.
  9. The local defences start working instantaneously, do not depend on previous exposure, and are effective against a wide range of infectious agents.
  10. When germs or their toxins reach the deeper tissues, the body fluids fight them with antibodies, which act on the germs, and antitoxins, which act on their poisons.
  11. Immunity is the resistance of the body to the onset of a disease after infection by harmful germs.
  12. Innate immunity is inborn and comes from the genetic make-up. It is either non-specific, giving general resistance to all infections, or genetically determined resistance to one particular germ.
  13. Acquired immunity is developed during an individual’s lifetime, and is either active or passive.
  14. Actively acquired immunity is produced by the body’s own lymphocytes on exposure to an antigen — naturally through a previous infection, or artificially through vaccination. It takes time to develop but is long-lasting, being carried by memory lymphocytes.
  15. Passively acquired immunity is supplied from outside as ready-made antibodies — naturally from the mother through the placenta and milk, or artificially as an antiserum prepared in a horse or other animal, as in antivenin for snake-bite.
  16. Antibodies are proteins of the immunoglobulin class, produced by specialised lymphocytes only on exposure to an antigen. They are specific, each acting on one antigen only, and the body can make an unlimited variety of them.
  17. Vaccination is the artificial introduction of germs or a germ substance into the body to develop resistance to a particular disease; the practice is called prophylaxis and the material is the vaccine. Immunisation is the resistance so developed.
  18. Vaccines are of four categories: killed germs, living weakened germs, living fully active germs of a related milder disease, and toxoids.
  19. The National Immunisation Schedule fixes the ages at which vaccines such as DPT, oral polio, BCG, measles, typhoid and tetanus toxoid are given, including booster doses and doses for mothers during pregnancy.
  20. A toxin is any poisonous substance produced by an animal, a plant or a bacterium. The substance formed against it was called an antitoxin, but the general term antibody is now used instead.
  21. Antiseptics are mild chemicals applied on the body to kill germs without harming it; disinfectants are strong chemicals applied on places where germs thrive, and must not touch the skin. Strong heat and boiling act as physical disinfectants. Deodorants are neither, and only mask a bad smell.
  22. Antibiotics are chemical substances produced by some microorganisms that kill or inhibit the growth of other microorganisms. Penicillin was discovered by Alexander Fleming in 1928 from the mould Penicillium, and the term antibiotics was coined by Selman Waksman in 1942.
  23. Penicillin comes from Penicillium notatum, with commercial production largely from Penicillium chrysogenum; streptomycin comes from the bacterium Streptomyces. Penicillin interferes with bacterial cell wall formation, streptomycin with protein synthesis at the ribosomes.
  24. Antibiotics are used to fight infections, as food preservatives, in animal feed and against plant pathogens.
  25. Chemotherapy is treatment by chemicals. Salvarsan, produced in 1910, killed the germs of syphilis but was often poisonous to the patient. The sulphonamides, discovered in the 1930s, are synthetic drugs that interfere with bacterial metabolism, and are now rarely used except along with antibiotics.

Flowchart

FC 17.6 — The Whole Defence, End To End Germ approaches the body → mechanical barriers, secretions and expulsion try to keep it out → if it enters the tissues, phagocytes engulf it without distinguishing its kind → lymphocytes recognise its antigen and produce specific antibodies and killer cells → germ rendered harmless and eliminated → memory lymphocytes retain the pattern → the same germ met again is destroyed before disease develops. Two shortcuts exist: vaccination supplies the antigen in advance so this record is built without the disease; an antiserum supplies ready-made antibodies from outside when there is no time to build one.

17.13 Key Terms

  1. Acquired immunity — resistance to a disease that an individual develops during his or her own lifetime, and was not born with.
  2. Antibiotic — a chemical substance produced by some microorganisms which kills or inhibits the growth of other microorganisms.
  3. Antibodies — proteins in the body fluids, produced by lymphocytes, which act against a particular germ or antigen.
  4. Antigen — a chemical found on the surface of a disease-causing germ, which makes the body produce antibodies against it.
  5. Antiseptics — mild chemicals applied on the body to kill germs without harming the skin or tissues.
  6. Antiserum — a preparation of serum containing ready-made antibodies, obtained from an animal and injected into a patient.
  7. Antitoxins — antibodies that act against the poisons, rather than the bodies, of germs.
  8. Antivenin — an antiserum containing ready-made antibodies against snake venom.
  9. Artificially acquired active immunity — immunity produced by the body itself after an antigen has been introduced deliberately, as by vaccination.
  10. Booster dose — a repeat dose of a vaccine given after the first course, to raise protection that has begun to fade.
  11. Chemotherapy — the treatment of disease by the use of chemicals.
  12. Cilia — tiny hair-like structures on a lining membrane which beat together and sweep mucus and trapped particles outwards.
  13. Diapedesis — the squeezing of a white blood cell through the wall of a blood capillary into the surrounding tissue.
  14. Disinfectants — strong chemicals applied on places where germs thrive, too harsh to be used on the human body.
  15. Germs — microbes capable of causing disease.
  16. Immunity — the resistance of the body to the onset of a disease after infection by harmful germs.
  17. Immunoglobulins — the class of proteins to which antibodies belong.
  18. Innate immunity — inborn immunity arising from the genetic make-up, present without any previous infection.
  19. Keratin — the tough protein of the outer layer of the skin, which makes it almost impermeable to germs.
  20. Killer cells — cells produced by lymphocytes that carry specific receptors for the antigens of invading germs.
  21. Lymphocytes — a type of white blood cell that produces antibodies and killer cells on exposure to an antigen.
  22. Mucous membrane — the soft moist lining of a passage that opens to the outside of the body.
  23. Mucus — the slimy secretion of an epithelial lining, which traps germs and dust.
  24. Naturally acquired active immunity — immunity produced by the body itself after a previous natural infection.
  25. Penicillin — the first antibiotic, obtained from the mould Penicillium, which prevents bacteria from forming their cell walls.
  26. Phagocytes — white blood cells that engulf and destroy germs.
  27. Phagocytosis — the engulfing and destroying of a germ by a phagocyte.
  28. Pollutants — poisonous chemicals in air, water or food that enter and harm the body.
  29. Prophylaxis — the scientific name for the practice of protecting against a disease in advance.
  30. Pus — a mixture of destroyed germs, killed white blood cells and damaged tissue cells at a wound.
  31. Sulphonamides — a group of synthetic drugs that kill bacteria by interfering with their metabolism.
  32. Toxins — poisonous substances produced by an animal, a plant or a bacterium.
  33. Toxoid — a bacterial toxin inactivated by dilute formalin, which can no longer poison but can still produce antibodies.
  34. Vaccination — the introduction of dead or weakened germs into the body to develop immunity against a disease.
  35. Vaccine — the material introduced into the body during vaccination.
  36. Venom — the toxin injected by a snake or similar animal.

17.14 Points To Ponder

  1. The local defences act instantly but treat every germ alike, while the immune system is precise but slow to get started. Would a body that had only one of the two be better off with the fast system or the accurate one?
  2. A vaccine works by producing an antigen that the body will remember. Why, then, does a person need a fresh vaccination against influenza in many years, but only one course against tetanus with occasional boosters?
  3. Passive immunity fades within weeks and gives no memory at all. Given that, why is it still worth giving to a person who has just been bitten by a snake — and why would it be a poor choice for protecting a whole village in advance?
  4. Smallpox was eradicated and its vaccination stopped altogether. What must be true about a disease for eradication to be possible at all, and why has the same not happened with the common cold?
  5. Antibiotics were found among microorganisms that compete with one another. If bacteria have been meeting such chemicals for millions of years, what does that suggest about how long any single antibiotic can go on working?
  6. A person’s skin, stomach acid and mucus all destroy microbes, yet the intestine is home to enormous numbers of bacteria that the body does not attack. What would the body need to be able to do for that arrangement to be possible?

17.15 Objective Questions

  1. The tough protein that makes the outer layer of the skin almost impermeable to germs is: (1 mark)
    1. mucus
    2. keratin
    3. immunoglobulin
    4. formalin
  2. The squeezing of a white blood cell through the wall of a blood capillary is called: (1 mark)
    1. phagocytosis
    2. prophylaxis
    3. diapedesis
    4. chemotherapy
  3. Antibodies received by a baby through its mother’s milk are an example of: (1 mark)
    1. naturally acquired active immunity
    2. artificially acquired active immunity
    3. naturally acquired passive immunity
    4. artificially acquired passive immunity
  4. A toxoid is prepared by treating a bacterial toxin with: (1 mark)
    1. dilute formalin
    2. carbolic acid
    3. bordeaux mixture
    4. streptomycin
  5. Which of the following is a vaccine made of living but weakened germs? (1 mark)
    1. TAB vaccine
    2. Salk’s vaccine
    3. BCG vaccine
    4. tetanus toxoid
  6. Penicillin acts on bacteria by interfering with: (1 mark)
    1. the formation of the cell wall
    2. the working of the ribosomes
    3. the entry of water into the cell
    4. the antigens on the cell surface
  7. Choose the correct option. (1 mark) Assertion (A): An antiserum acts immediately after it is injected. Reason (R): An antiserum supplies antibodies that the patient’s own body has not had to make.
    1. Both A and R are true, and R is the correct explanation of A.
    2. Both A and R are true, but R is not the correct explanation of A.
    3. A is true but R is false.
    4. A is false but R is true.
  8. Choose the correct option. (1 mark) Assertion (A): The local defence system is effective against a wide range of infectious agents. Reason (R): The local defence system recognises the antigen of each germ before acting on it.
    1. Both A and R are true, and R is the correct explanation of A.
    2. Both A and R are true, but R is not the correct explanation of A.
    3. A is true but R is false.
    4. A is false but R is true.
  9. Choose the correct option. (1 mark) Assertion (A): One kind of antibody cannot protect against every disease. Reason (R): An antibody binds only to the particular antigen it was produced against.
    1. Both A and R are true, and R is the correct explanation of A.
    2. Both A and R are true, but R is not the correct explanation of A.
    3. A is true but R is false.
    4. A is false but R is true.
  10. Match each item in Column A with the correct item in Column B. (5 marks)
Column A Column B
a. Salk’s vaccine i. mould from which penicillin was first obtained
b. Penicillium notatum ii. coined the term antibiotics
c. Selman Waksman iii. strong chemical applied on places, not on the body
d. Phenol iv. poliomyelitis
e. Cilia v. sweep trapped mucus out of the windpipe
  1. Fill in the blanks. (4 marks)
    1. The slimy secretion of an epithelial lining that traps bacteria is called ____.
    2. Immunity that is present from birth by virtue of the genetic make-up is called ____ immunity.
    3. The scientific name for the practice of protecting a person against a disease in advance is ____.
    4. A repeat dose of a vaccine given to lift protection that has begun to fade is called a ____ dose.
  2. State whether each statement is true or false. (4 marks)
    1. Deodorants are mild disinfectants.
    2. Antitoxins act on the poisons released by germs.
    3. Passive immunity lasts longer than active immunity.
    4. Boiling can be described as a physical means of disinfection.
  3. Name the following. (5 marks)
    1. The acid secreted by the stomach that kills swallowed germs.
    2. The class of proteins to which antibodies belong.
    3. The white blood cells that produce antibodies and killer cells.
    4. The bacterium from which streptomycin is obtained.
    5. The group of synthetic drugs that kill bacteria by interfering with their metabolism.
  4. In each set, pick the odd one out and give a reason for your choice. (4 marks)
    1. Skin, hair, mucus, antibody
    2. Iodine, boric acid, phenol, mercurochrome
    3. Coughing, sneezing, vomiting, phagocytosis
    4. DPT, BCG, TAB, DDT

17.16 Check Your Concepts

  1. Define immunity. (2 marks)

  2. State the four ways by which germs may enter the human body. (2 marks)

  3. Give a reason: a cut in the skin is more dangerous than germs resting on unbroken skin. (2 marks)

  4. What is pus, and of what is it made? (2 marks)

  5. Give a reason: a person who has had measles once does not usually suffer from it again. (2 marks)

  6. State three merits of the local defence system. (3 marks)

  7. Differentiate between an antiseptic and a disinfectant, stating the basis of difference. (3 marks)

  8. Explain, with one example, what is meant by a toxoid. (3 marks)

  9. Why does a vaccine made of killed germs still protect a person against the living germ? (3 marks)

  10. Describe how a phagocyte reaches a germ that is multiplying in a tissue, and what it then does to it. (4 marks)

  11. Differentiate between actively acquired and passively acquired immunity, stating the basis of difference, and give one example of each. (4 marks)

  12. What would happen if the cilia lining the windpipe stopped beating? Explain your answer. (4 marks)

  13. Describe the four categories of vaccines, giving one example of each. (5 marks)

  14. Explain how penicillin was discovered, stating clearly what was observed and what was concluded from that observation. (5 marks)

17.17 Figure-Based Questions

  1. Study Fig. 17.2-1. Name the four routes marked, and state which one is guarded by hydrochloric acid. (3 marks)

  2. Fig. 17.3-2 shows three stages, A, B and C. Name the process shown in stage A and the process shown in stage C, and state one function of the cell drawn in all three stages. (3 marks)

  3. Look at Fig. 17.6-1. Name the cell that recognises the bacteria as foreign, name the substance it secretes, and state what that substance does to the bacteria before the white cell arrives. (3 marks)

  4. Fig. 17.5-1 shows four stages in preparing a treatment. Name the fluid drawn from the animal’s blood, and state why the patient’s own body is not made to produce the antibodies instead. (3 marks)

  5. Redraw Fig. 17.10-1 and label the mould colony, the clear zone and the bacterial colonies. State what the clear zone shows. (4 marks)