Diseases and immunity

A pathogen 病原体 is an organism that causes disease 疾病. A transmissible disease 传染病 is one in which the pathogen can pass from one host 宿主 to another.

A pathogen is transmitted 传播 in two main ways:

• by direct contact 直接接触 — for example through blood and other body fluids 体液.
• indirectly — for example from contaminated 污染 surfaces, food, animals, or the air.

Your body has several defences 防御 that keep pathogens out or destroy them:

• skin — a barrier that covers and protects the body.
• hairs in the nose — trap dust and pathogens in the air you breathe in.
• mucus 黏液 — sticky liquid in the airways that traps pathogens.
• stomach acid 胃酸 — kills most pathogens in your food.
• white blood cells 白细胞 — find and destroy any pathogens that get inside.

The skin is the first barrier: pathogens cannot pass through unbroken skin. When the skin is cut, the blood clots, sealing the wound quickly so pathogens cannot enter. Skin, nose hairs, mucus and stomach acid are mechanical or chemical barriers that work on every pathogen the same way. White blood cells are the defence inside the body: phagocytes engulf and digest pathogens, and (Supplement) lymphocytes make antibodies that target one kind of pathogen.

Good public health stops disease spreading:

• a clean water supply, so drinking water carries no pathogens.
• hygienic 卫生 food preparation and good personal hygiene, such as washing your hands.
• proper waste disposal and sewage treatment 污水处理, so waste does not contaminate water or food.

Exam questions ask you to explain why each one matters, so link it to how the pathogen spreads:

Antigens and antibodies

Every pathogen carries antigens 抗原 on its surface, and each kind of antigen has its own special shape.

Antibodies 抗体 are proteins 蛋白质 made by lymphocytes 淋巴细胞 (a kind of white blood cell). An antibody has a shape that is complementary 互补 to one antigen, so it fits only that antigen. Binding can neutralise pathogens or their toxins, or mark pathogens so that phagocytes 吞噬细胞 can engulf and destroy them.

Active immunity and vaccination

Active immunity 主动免疫 is protection made by your own body producing antibodies. You gain it after an infection, or after vaccination 疫苗接种.

Vaccination works like this:

1. weakened pathogens, or just their antigens, are put into the body; they cannot make you ill.
2. the antigens make your lymphocytes produce antibodies.
3. the body also makes memory cells 记忆细胞 that stay for years.

If the real pathogen enters later, the memory cells make antibodies very fast, so you do not become ill. This gives long-term protection. If most people in a group are vaccinated, the disease cannot spread easily.

How vaccination controls the spread of a disease. A vaccinated person does not become ill, so they do not pass the pathogen on. When a large proportion of a population is vaccinated, an infected person meets mostly immune people, so the pathogen cannot find new hosts and the chain of transmission is broken. This also protects the few people who cannot be vaccinated (such as newborn babies, or people who are very ill). This is called herd immunity 群体免疫. Smallpox was eradicated 根除 from the whole world in 1980 by vaccinating everyone around each case.

Worked example. After a first dose of a vaccine, antibody levels rise slowly and stay low. After a second dose of the same vaccine, they rise faster and reach a much higher level. Explain. At the first dose the body has no memory cells for that antigen, so time is lost while the right lymphocyte is found and multiplies - the response is slow and small. The second dose meets the memory cells left over from the first, which recognise the antigen at once, so antibodies are made sooner and in far greater numbers. The marks are for memory cells: "the body is used to it" earns nothing.

Passive immunity

Passive immunity 被动免疫 is protection from antibodies made by another body, not your own. For example, a baby receives antibodies from its mother across the placenta 胎盘 and in breast milk 母乳. This is why breast-feeding 母乳喂养 helps protect infants 婴儿. Passive immunity is only short-term, because no memory cells are made.

Why breast-feeding matters. A newborn baby's immune system is not yet fully developed, so it cannot make many antibodies of its own. The antibodies in breast milk protect it against the pathogens the mother has met — which are the ones the baby is most likely to meet too — until its own immune system can respond. Antibodies can also be injected, for example after a snake bite or an exposure to tetanus, to give instant but short-term protection.

Cholera 霍乱 is a disease caused by a bacterium 细菌 that spreads in contaminated water. The cholera bacterium makes a toxin 毒素. This toxin causes chloride ions 氯离子 to be pumped into the small intestine 小肠. Water then moves into the intestine by osmosis 渗透. The result is diarrhoea 腹泻, dehydration 脱水 (loss of water) and a loss of ions 离子 from the blood.

Worked example. Explain why a person with cholera can die within a day, and how they can be treated. The toxin causes chloride ions to be secreted into the small intestine, lowering the water potential there, so water moves from the blood into the gut by osmosis and is lost as watery diarrhoea. The person loses so much water and so many ions from the blood that they become dangerously dehydrated. The treatment is to replace them: drinking oral rehydration solution 口服补液 (water with salts and glucose), or a drip into a vein if very ill. Cholera is prevented by a clean water supply and sewage treatment.

Common mistakes.

• Saying a vaccine contains antibodies. A vaccine contains weakened pathogens or their antigens; your own lymphocytes then make the antibodies and memory cells.
• Writing just "white blood cells" when the question asks which cells. Lymphocytes make antibodies; phagocytes engulf and digest pathogens. Name the right one.
• Saying memory cells make you immune because they "remember the disease". The marks are for: memory cells recognise the antigen and quickly make many antibodies, so the pathogen is destroyed before you become ill.
• Saying an antibody works against any pathogen. Its shape is complementary to one antigen, so a new antibody is needed for each kind of pathogen.
• Calling a baby's protection from breast milk long-term. The antibodies come from the mother and no memory cells are made, so passive immunity lasts only a short time.
• Saying cholera bacteria pump out water. The toxin makes chloride ions move into the small intestine, and water follows by osmosis.

• Pathogen = organism that causes disease. Transmissible = can pass between hosts, by direct contact or indirectly.
• Five body defences: skin, nose hairs, mucus, stomach acid, white blood cells.
• An antibody's shape is complementary to one antigen (like an enzyme and its substrate).
• Active immunity (from infection or vaccination) makes memory cells → long-term. Passive immunity (placenta, breast milk) makes no memory cells → short-term.
• Cholera: toxin → chloride into the gut → water by osmosis → diarrhoea and dehydration.
• For each public-health measure, say which route of spread it breaks (water, food, hands, flies, sewage).
• Vaccination controls spread because vaccinated people do not become ill and cannot pass the pathogen on; if most are vaccinated, the chain of transmission breaks.