Human Health and Disease — Pathogens, the Immune System and Immunity
This chapter has two very different halves, and it helps to treat them separately. The first is a table of diseases — pathogen, name, and how it spreads — which is pure recall and easy marks if you drill the pairings. The second is the immune system, which is conceptual and rewards understanding: the difference between innate and acquired immunity, between B-cells and T-cells, and between active and passive immunity. The single most examined idea is the distinction between active immunity (you make your own antibodies, slow but lasting) and passive immunity (antibodies given ready-made, fast but temporary) — a vaccine versus an antivenom. Learn the disease table by rote and the immunity logic by reasoning, and the chapter is very scoring.
The Immune System
- Innate immunity is non-specific and present from birth — physical barriers (skin, mucus), phagocytes, and the inflammatory response. Acquired (adaptive) immunity is specific, develops after exposure, and remembers the pathogen.
- Acquired immunity has two arms: humoral (antibody-mediated), run by B-lymphocytes that secrete antibodies into the blood; and cell-mediated immunity (CMI), run by T-lymphocytes that act directly on infected cells and drive graft rejection.
- An antibody is a Y-shaped protein of four chains — two heavy, two light, written H₂L₂. The five classes are IgG, IgA, IgM, IgE, IgD; IgG is most abundant and crosses the placenta, IgA is in secretions (milk, saliva), IgE mediates allergy.
🧠 Active versus passive immunity — who makes the antibodies?
The whole distinction turns on one question: does the body make its own antibodies, or receive them ready-made? In active immunity the body is exposed to an antigen and makes its own antibodies — either by catching the disease or by a vaccine, which is a harmless dose of the antigen. It is slow to develop but long-lasting, because it also builds memory cells. In passive immunity, ready-made antibodies are given directly — as in an antivenom after a snakebite, or the mother's antibodies reaching the foetus across the placenta and the baby through breast milk. It is fast-acting but short-lived, because the body did not make the antibodies and keeps no memory. So the test to apply is simple: if the person's own immune system did the work (disease, vaccine), it is active; if the antibodies came from outside (antivenom, mother's milk, injected antibodies), it is passive. A vaccine is the classic active example precisely because it makes you produce the response; an antivenom is the classic passive example precisely because there is no time to wait for you to.AIDS, Cancer and Addiction
- AIDS is caused by HIV, a retrovirus that infects helper T-cells, so the immune system itself is crippled. It spreads by unprotected sex, infected blood, shared needles and mother to child — not by casual contact. Diagnosed by the ELISA test.
- Cancer is uncontrolled cell division; cells lose contact inhibition and may spread (metastasis). Causes (carcinogens) include tobacco, radiation and oncogenic viruses. Benign tumours stay put; malignant ones invade and metastasise.
- Drugs and alcohol: opioids (heroin/smack, from the poppy) are depressants; cannabinoids (from Cannabis) affect the heart; cocaine ("coke") is a stimulant. Dependence has psychological and physiological (withdrawal) components.
🎯 Why HIV is uniquely devastating — it attacks the defence itself
Most pathogens are dangerous because they damage a particular tissue; HIV is dangerous for a subtler reason — it attacks the immune system that is supposed to fight it. HIV is a retrovirus, meaning it carries RNA and uses the enzyme reverse transcriptase to make a DNA copy that integrates into the host genome. Crucially, the cells it infects and destroys are the helper T-cells, the very cells that coordinate the immune response. As their numbers fall, the whole immune system weakens, and the person becomes unable to fight off infections that a healthy body shrugs off. That is why AIDS patients ultimately die of ordinary opportunistic infections rather than of HIV directly: the virus does not kill you, it removes your ability to stop anything else from killing you. Two exam points follow. First, this explains the long latency — the immune collapse builds gradually as T-cells are depleted. Second, it explains why AIDS spreads only through blood, sexual fluids, shared needles and mother-to-child, and not through casual contact, sharing food, or insect bites — a public-health fact the exam states plainly to counter the stigma.Key takeaways
- Malaria = Plasmodium (female Anopheles); typhoid = Salmonella (Widal test); pneumonia = Streptococcus.
- Ringworm is caused by FUNGI, not a worm, despite the name; filariasis = Wuchereria via mosquito.
- Innate immunity = non-specific, from birth; acquired immunity = specific, has memory.
- B-cells → humoral (antibody) immunity; T-cells → cell-mediated immunity and graft rejection.
- Active immunity: body makes its own antibodies (vaccine/disease), slow but lasting. Passive: ready-made antibodies (antivenom, mother's milk), fast but temporary.
- Antibody = H₂L₂ (two heavy + two light chains); IgG most abundant and crosses placenta; IgE in allergy.
- HIV (retrovirus) infects helper T-cells → immune collapse; diagnosed by ELISA; not spread by casual contact.
- Cancer: uncontrolled division, loss of contact inhibition; benign stays, malignant metastasises.
Practice questions (8)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
Ringworm, despite its name, is caused by:
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Answer: A — Fungi such as Microsporum and Trichophyton
Ringworm is a fungal skin infection caused by genera such as Microsporum and Trichophyton; the "worm" in the name refers only to the ring-shaped rash. Reaching for a roundworm because of the name is exactly the trap this question is set to catch.Injecting a snakebite victim with ready-made antibodies (antivenom) confers:
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Answer: A — Passive immunity — fast-acting but short-lived
Because the antibodies are given ready-made rather than produced by the body, this is passive immunity — quick to act but temporary, with no memory. Active immunity would require the body to make its own antibodies, which takes too long for an emergency like a snakebite.HIV, the cause of AIDS, primarily infects and destroys:
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Answer: A — Helper T-lymphocytes
HIV targets helper T-cells, the coordinators of the immune response, so their loss cripples immunity and lets opportunistic infections take hold. Because it disables the defence rather than a single organ, patients die of secondary infections, which is why the T-cell answer is central.A vaccine produces immunity by:
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Answer: A — Exposing the body to an antigen so it makes its own antibodies and memory cells
A vaccine is a harmless form of the antigen that prompts the body to make its own antibodies and memory cells — active immunity, slow but durable. Supplying ready-made antibodies is passive immunity, the opposite mechanism, so that option describes an antivenom rather than a vaccine.Cell-mediated immunity and the rejection of transplanted grafts are carried out mainly by:
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Answer: A — T-lymphocytes
T-lymphocytes act directly on infected or foreign cells, driving cell-mediated immunity and graft rejection. B-lymphocytes instead secrete antibodies for humoral immunity, so assigning graft rejection to them confuses the two arms of acquired immunity.Malaria is transmitted to humans by the bite of the:
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Answer: A — Female Anopheles mosquito
Only the female Anopheles takes blood meals and transmits Plasmodium, so it is the malaria vector. Culex spreads filariasis and the male mosquito feeds on nectar, so those options name the wrong vector or the wrong sex.A malignant tumour differs from a benign one chiefly in that malignant cells:
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Answer: A — Invade surrounding tissue and spread (metastasise)
Malignant cells lose contact inhibition, invade neighbouring tissue and metastasise to distant sites, which is what makes them dangerous. Benign tumours stay confined, so "remain confined" describes the benign type the question is contrasting against.Which antibody class is the most abundant in blood and is able to cross the placenta to the foetus?
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Answer: A — IgG
IgG is the most plentiful antibody in blood and the only class that crosses the placenta, giving the newborn passive immunity. IgE mediates allergy and IgA is found in secretions like milk and saliva, so neither fits the "abundant and placenta-crossing" description.