Disease and Immunity

• Pathogen, route, contact, entry, response. Five links — and disease control is nothing more than breaking one of them.
• Eggs, larva, pupa, adult. Water, water, water, air. Take away the water and three quarters of the life cycle has nowhere to happen.
• If you can only remember one repair from this page, make it this: the pathogen owns the antigen, your lymphocyte makes the antibody, your phagocyte does the eating, and the antibiotic is a medicine that never meets a virus.
• Four chains, nine definitions, one diagram. That is the whole topic, and it fits on a single sheet of paper.

• Define pathogen and transmissible disease, and use both terms correctly in the same sentence.
• Classify a described route of transmission as direct or indirect, and justify the classification from the route itself.
• Describe the body's first-line barriers — skin, nasal hairs, mucus, cilia, stomach acid, clotting — and state honestly what each one cannot do.
• Explain the transmission of malaria in terms of a parasite as the pathogen and a mosquito as the vector, never the other way round.
• Link every malaria control measure to the stage of the mosquito life cycle, or the stage of transmission, that it attacks.
• State how HIV is transmitted, state how it is not transmitted, and explain how it weakens the immune system and may progress to AIDS.
• Give the cholera mechanism as a complete causal chain using the words toxin, chloride ions, water potential and osmosis.
• Describe the effects of excessive alcohol consumption, and the effects of nicotine, tar and carbon monoxide in tobacco smoke.
• Define drug; explain why antibiotics treat bacterial infections but not viral ones.
• Explain the appearance of an antibiotic-resistant strain such as MRSA as natural selection acting on random mutation.
• Explain the roles of antigens, antibodies, lymphocytes and phagocytes, keeping each one in its own job.
• Describe vaccination as a sequence from antigen to memory cell to a faster secondary response, and read a primary-versus-secondary antibody graph.
• Compare active and passive immunity on source, speed, duration and memory cells, and place breast-feeding correctly in that comparison.

Why you often feel nothing the second time. During the primary response the pathogen has days in which to reproduce before enough antibody exists, which is why you become ill. During the secondary response antibody appears so quickly that the pathogen is often destroyed before it has reproduced enough to cause symptoms. The immunity is not that the pathogen cannot enter; it is that it does not get far.

Pathogen

A disease-causing organism. Bacteria, viruses, fungi and parasites can all be pathogens; the term describes the role an organism plays in causing disease, not the group it belongs to, and it does not include an organism that merely carries a pathogen from one host to another.

HIV

Human immunodeficiency virus, a viral pathogen transmitted in infected sexual fluids and blood, through shared contaminated needles, and from a mother to her child during pregnancy, birth or breast-feeding. It infects and reduces the lymphocytes responsible for antibody production, so immune defence weakens over time.

Direct Transmission

Transmission in which a pathogen passes straight from an infected host to a new host with nothing in between, for example by physical contact, sexual contact, infected blood, other infected body fluids, or from a mother to her child during pregnancy, birth or breast-feeding.

First Line Of Defence

The body's physical and chemical barriers to pathogen entry, acting before any specific immune response: intact skin, hairs in the nose, mucus secreted by goblet cells, cilia that sweep mucus towards the throat, acid conditions in the stomach, and clotting that seals a break in the skin. These barriers reduce the number of pathogens that enter but do not prevent all infection.

Transmissible Disease

A disease in which the pathogen can be passed from one host to another. The definition depends on whether the pathogen can move between hosts, not on how severe the illness is, so many serious diseases are not transmissible and some mild ones are.

Indirect Transmission

Transmission in which a pathogen reaches a new host by way of something else: contaminated food, water or surfaces, droplets carried in the air, shared contaminated equipment, or an animal vector that carries the pathogen from one host to another.

Cholera Toxin

A substance produced by cholera bacteria in the small intestine that causes chloride ions to be secreted from the intestinal cells into the lumen. The resulting fall in the water potential of the lumen makes water move out of the blood and tissues into the intestine by osmosis, producing severe watery diarrhoea.

Antiretroviral Treatment

Medicines given to a person infected with HIV that reduce the reproduction of the virus in the body. Treatment slows the loss of lymphocytes and helps the immune system keep working for longer, and by reducing the amount of virus present it also lowers the chance of transmission; it is not a cure and does not remove the virus from the body.

Route Of Transmission

The particular path a pathogen takes from an infected host to a new host. Naming the route matters because every control measure works by interrupting one specific step in it, and an explanation that does not identify the step interrupted is incomplete.

Mosquito Life Cycle

The four stages through which a mosquito develops: eggs laid on the surface of water, an aquatic larva, an aquatic pupa, and a flying adult. Three of the four stages depend on standing water, which is why removing or covering standing water removes the breeding site and reduces the number of adult mosquitoes that emerge.

Depressant

A substance that slows the activity of the nervous system. Alcohol acts as a depressant, which is why excessive consumption reduces self-control, slows reaction time and impairs judgement and coordination; the term refers to slowed nervous activity, not to low mood.

Cholera

A transmissible disease caused by a bacterium, transmitted mainly in drinking water contaminated with faecal material from an infected person. The bacterium produces a toxin in the small intestine that causes severe watery diarrhoea, and the resulting loss of water and ions from the blood is what makes the disease dangerous.

Vector

An organism that transmits a pathogen from one host to another. A mosquito is the vector for the malarial parasite: it carries the parasite between people but is not itself the cause of the disease, which is why control of malaria means control of the mosquito and of the parasite in people, not of one alone.

AIDS

Acquired immune deficiency syndrome: the condition that may develop after HIV has reduced the number and effectiveness of lymphocytes to the point where the immune system can no longer defend the body properly, so susceptibility to other infections and to some diseases is greatly increased. AIDS is a stage that may follow HIV infection, not another name for the virus.

Carbon Monoxide

A gas present in tobacco smoke that binds strongly to haemoglobin in red blood cells. Haemoglobin that has bound carbon monoxide cannot carry oxygen, so the oxygen-carrying capacity of the blood is reduced, the heart is placed under greater strain, and during pregnancy the oxygen supply to the fetus is reduced.

MRSA

Methicillin-resistant Staphylococcus aureus, a strain of bacteria that is not killed by several commonly used antibiotics. It is the standard example of a resistant strain arising by natural selection, and infections caused by it are harder to treat because fewer antibiotics remain effective against it.

Antibiotic

A medicine that acts against bacteria. Different antibiotics act on structures or processes that bacteria possess, such as the bacterial cell wall or bacterial ribosomes, and which the cells of the person taking them do not share in the same form; they have no effect on viruses.

Faecal-Oral Route

A route of transmission in which pathogens from the gut of an infected person reach the mouth of another, usually by way of contaminated water, food, hands or surfaces. It is interrupted at the source by safe disposal of faeces and sewage treatment, and near the mouth by treated drinking water, handwashing and hygienic food preparation.

Drug

Any substance taken into the body that modifies or affects chemical reactions in the body. The definition is deliberately wide: it includes medicines such as antibiotics as well as substances such as alcohol and nicotine, and it says nothing about whether a substance is legal or harmful.

Vector Control

Any measure that reduces the transmission of a disease by acting on the organism that carries the pathogen. For malaria this includes removing the standing water in which eggs, larvae and pupae develop, killing adult mosquitoes with insecticides, and preventing bites with treated bed nets, window screens and protective clothing.

Emphysema

A lung condition in which the walls of the alveoli are damaged and break down, so that many small air sacs are replaced by fewer, larger spaces. This greatly reduces the total surface area available for gas exchange, so less oxygen can diffuse into the blood and breathlessness results.

Passive Immunity

Short-term defence against a pathogen provided by antibodies obtained from another individual. Because the antibodies are ready-made, protection is immediate, but the recipient's own lymphocytes have not responded and no corresponding memory cells are formed, so protection declines as the transferred antibodies are broken down.

Antibody

A protein produced by lymphocytes that has a binding site complementary in shape to one particular antigen. Antibodies bind specifically to that antigen, which can cause pathogens to clump together, neutralise a toxin, or mark the pathogen so that phagocytes engulf it; antibodies do not engulf pathogens themselves.

Antigen

A molecule, usually on the surface of a pathogen, that is recognised as foreign and stimulates a specific immune response. Different pathogens carry antigens of different shapes, which is why the antibody produced against one pathogen does not act against another.

Active Immunity

Defence against a pathogen by antibody production in the person's own body. It develops after infection with the pathogen or after vaccination, involves the person's own lymphocytes, and produces memory cells, so protection is usually long lasting although not necessarily lifelong.

Vaccination

The introduction of pathogen antigens into the body, in a form that does not cause the disease, so that specific lymphocytes respond, antibodies are produced and memory cells are formed. Vaccination therefore stimulates active immunity; it does not supply ready-made antibodies, and it is preventive rather than a treatment for an infection already present.

Memory Cell

A lymphocyte that remains in the body after an immune response to a particular antigen. Memory cells persist long after the antibodies themselves have disappeared, and on a second exposure to the same antigen they allow antibodies to be produced far more quickly and in far greater quantity, so the pathogen is often destroyed before symptoms develop.

Population Immunity

The reduction in transmission of a disease that follows when a high proportion of a population is immune. With fewer susceptible hosts available, chains of transmission are interrupted more often, which indirectly protects some people who cannot be vaccinated or who do not develop full immunity; it does not make any unvaccinated individual immune.

Phagocyte

A white blood cell that engulfs and digests pathogens, a process called phagocytosis. Phagocytes act non-specifically against many kinds of pathogen and do not produce antibodies; their work is made more effective when antibodies have already bound to a pathogen and marked it or caused pathogens to clump together.

• "Every disease is transmissible." Transmissible means a pathogen can pass from one host to another. Diseases caused by diet, by inherited alleles or by the body's own processes have no pathogen to pass. Write A transmissible disease is one in which the pathogen can be passed from one host to another; many diseases are not transmissible.
• "The mosquito causes malaria." The mosquito is the vector. A parasite is the pathogen. Naming the wrong organism as the cause loses the mark even when the rest of the transmission chain is correct. Write Malaria is caused by a parasite, which is transmitted from person to person by a mosquito acting as a vector.
• "Malaria can spread by touching someone who has it." There is no such route. Natural transmission goes through the mosquito, and an answer that abandons the vector has abandoned the mechanism. Write Malaria is transmitted only when a mosquito that has fed on an infected person later feeds on another person.
• "HIV and AIDS are two names for the same thing." One is a virus; the other is a condition that may develop after that virus has weakened the immune system over time. Write HIV is the virus. AIDS is the condition in which the immune system is so weakened that susceptibility to other infections and diseases is greatly increased.
• "HIV can be spread by sharing utensils or by an insect bite." Neither moves infected blood or sexual fluids into another person's bloodstream, and that is the only way the virus is transmitted. Write HIV is transmitted in infected sexual fluids or blood, through shared contaminated needles, or from mother to child. Ordinary social contact does not transmit it.
• "Antiretroviral treatment cures HIV." It reduces the reproduction of the virus in the body. The virus is not removed. Write Treatment reduces the amount of virus, slows the loss of lymphocytes and lowers the chance of transmission; it is not a cure.
• "Cholera diarrhoea happens because water is not absorbed." That describes a different and far milder situation. In cholera, water is actively drawn out of the blood and tissues into the gut. Write The toxin causes chloride ions to be secreted into the lumen; the water potential of the lumen falls; water moves in from the blood and tissues by osmosis.
• "Cholera bacteria make holes in the intestine wall." The wall stays intact. What changes is what it transports. Write The bacteria produce a toxin that changes the movement of chloride ions across the intestinal cells.
• "Nicotine causes lung cancer." The carcinogens are in the tar. Nicotine is the addictive component and acts on the cardiovascular system. Write Tar contains carcinogens and is associated with lung cancer; nicotine is addictive and contributes to the risk of coronary heart disease.
• "Carbon monoxide destroys haemoglobin." It binds to haemoglobin. Haemoglobin that has bound carbon monoxide cannot carry oxygen, which is a different statement from being destroyed. Write Carbon monoxide binds strongly to haemoglobin, so less oxygen is transported and the heart works harder.
• "Every smoker gets lung cancer." The relationship is an increased risk across a population, not a certainty for an individual. Write Smoking is strongly associated with an increased risk of bronchitis, emphysema, lung cancer and coronary heart disease.
• "Antibiotics kill viruses." Antibiotics act on bacterial structures and processes. Viruses have none of them. Write Antibiotics act against bacteria only. Viruses lack the bacterial structures and independent processes that antibiotics target.
• "The person becomes resistant to the antibiotic." People do not become antibiotic-resistant. The bacteria do. Write The population of bacteria becomes resistant, so the antibiotic is less effective against infections caused by that strain.
• "The antibiotic made the bacteria mutate." Mutation is random and happens whether or not the antibiotic is present. The antibiotic selects; it does not cause. Write Random mutation had already produced resistant bacteria; the antibiotic then killed the bacteria that were not resistant.
• "Individual bacteria adapt to survive the antibiotic." An individual bacterium does not change during its lifetime in response to a need. The population changes because different individuals survive at different rates. Write Resistant bacteria survive and reproduce, passing resistance alleles to their offspring, so the resistant strain becomes more common.
• "Antibodies engulf pathogens." Antibodies are proteins. A protein cannot surround and swallow anything. Phagocytes are cells, and they engulf. Write Antibodies bind to antigens, which can clump pathogens together or mark them so that phagocytes engulf and digest them.
• "Phagocytes produce specific antibodies." Phagocytes are non-specific and do not produce antibodies. Lymphocytes do. Write Lymphocytes produce specific antibodies and form memory cells; phagocytes engulf and digest pathogens.
• "Antigens are made by lymphocytes." Antigens belong to the pathogen. Lymphocytes make antibodies against them. Write An antigen is a molecule, usually on the pathogen's surface, that stimulates a specific immune response.
• "Vaccination gives passive immunity." A vaccine supplies antigens, not antibodies, and your own lymphocytes do the work. Write Vaccination stimulates active immunity: the person's own lymphocytes produce antibodies and memory cells are formed.
• "Vaccines always contain live pathogens." They may contain weakened pathogens, inactivated pathogens, or the antigens alone. Write A vaccine contains antigens from the pathogen, in a form that does not cause the disease.
• "Passive immunity produces memory cells." The recipient's own lymphocytes never met the antigen, so there is nothing to remember. Write Passive immunity gives immediate protection but no memory cells are formed, so protection is short-term.
• "Breast milk gives a baby permanent immunity." The antibodies are received, not made, and they are broken down over time. Write Antibodies in breast milk give passive immunity: immediate but temporary protection while the infant's own immune system is developing.
• "If enough people are vaccinated, nobody can catch the disease." High coverage reduces transmission. It does not make an unvaccinated person immune, and it does not guarantee zero transmission. Write High vaccination coverage reduces the number of susceptible hosts, so chains of transmission are interrupted more often, indirectly protecting some people who cannot be vaccinated.
• "Skin and mucus stop all pathogens getting in." If they did, nobody would ever be ill. Barriers reduce entry; they do not prevent it. Write First-line barriers reduce the number of pathogens that enter the body, but they do not prevent all infection.

• This topic is unusual: almost none of it is calculation, and almost all of it is sequence. Examiners award marks for links in a chain, in order, with the correct vocabulary at each link. A paragraph that contains all the right words in the wrong order can score two marks out of five.
• Distractors are built from the classic confusions: antibody against antigen, vector against pathogen, active against passive. Read the stem twice and identify which pair is being tested before you look at the options.
• One or two marks. Definitions of pathogen, transmissible disease, drug and active immunity are asked directly. Learn them as sentences, not as ideas you will assemble on the day.
• Four to six marks. Cholera, antibiotic resistance and vaccination are the three favourites. Each is a numbered chain in this chapter. Write it as a chain and the marks fall out.
• A graph or table on vaccination coverage, resistance over time, or smoking and disease. Describe the trend, quote figures with units, then be careful: association is not proof of cause.
• Command word watch. State and describe want the observation. Explain wants the reason, and in this topic the reason is almost always a mechanism with at least three linked steps. Suggest means the answer is not printed anywhere in this chapter — you are being asked to apply the chain to an unfamiliar disease.