Immunity — Set 4

Correct Answer: D. Edward Jenner

• **Edward Jenner** = English physician who, in 1796, demonstrated that inoculation with cowpox material (a mild disease) could protect a person against the deadly smallpox virus. • **Foundation of modern immunology** — Jenner's work laid the scientific groundwork for vaccination as a concept; the word 'vaccine' itself comes from 'vacca' (Latin for cow) in his honour. • His method of using a related but weaker pathogen to train immunity is still the principle behind many vaccines today. • 💡 Option A (Louis Pasteur) is wrong because Pasteur developed vaccines for rabies and anthrax, not smallpox; Option B (Robert Koch) is wrong because Koch discovered the bacteria causing tuberculosis and cholera, not vaccines; Option C (Alexander Fleming) is wrong because Fleming discovered the antibiotic penicillin, which fights bacteria, not viruses.

Correct Answer: B. Tonsils

• **Tonsils** = clusters of lymphoid tissue located at the back of the throat (palatine tonsils) that serve as the first line of immune defence against pathogens entering through the nose and mouth. • **Sentinel function** — Tonsils sample antigens from inhaled or swallowed material and initiate an immune response before pathogens can penetrate deeper into the body. • They contain B-cells and T-cells ready to mount a rapid response upon detecting foreign material. • 💡 Option A (Liver) is wrong because the liver is a digestive and metabolic organ, not a lymphoid organ in the throat; Option C (Peyer's patches) is wrong because they are lymphoid patches in the small intestine wall, not the throat; Option D (Appendix) is wrong because the appendix is in the large intestine and plays a minor immune role in the gut, not the throat.

Correct Answer: D. A substance that triggers an immune response

• **A substance that triggers an immune response** = An antigen is any molecule — usually a protein or polysaccharide — found on the surface of pathogens (bacteria, viruses, fungi) that the immune system recognises as 'foreign' or 'non-self'. • **Antibody generation** — When an antigen is detected, B-cells produce specific antibodies shaped to bind that antigen, neutralising or marking the pathogen for destruction. • The term 'antigen' is short for 'antibody generator', highlighting its key role in triggering the adaptive immune response. • 💡 Option A (A part of the bone marrow) is wrong because bone marrow is the site of blood cell production, not an antigen; Option B (A medicine for pain) is wrong because pain medicines are analgesics, entirely unrelated to immunology; Option C (A type of white blood cell) is wrong because white blood cells (leukocytes) are the immune cells that respond to antigens, not the antigens themselves.

Correct Answer: C. IgA

• **IgA** = a secretory antibody that provides localised protection on the mucosal surfaces of the body — the lining of the gut, respiratory tract, eyes, and mouth. • **Passive immunity via breast milk** — IgA is the dominant antibody in colostrum (early breast milk), transferring maternal immunity to the newborn and protecting the infant's gut from pathogens. • IgA works by preventing pathogens from adhering to and colonising mucosal surfaces, essentially blocking entry before an infection can begin. • 💡 Option A (IgG) is wrong because IgG is the most abundant antibody in blood circulation and is not the primary secretory antibody; Option B (IgD) is wrong because IgD is found on the surface of B-cells and plays a role in B-cell activation, not secretions; Option D (IgE) is wrong because IgE is associated with allergic reactions and anti-parasitic responses, not mucosal secretions.

Correct Answer: B. General protection when most of a population is immune

• **General protection when most of a population is immune** = Herd immunity (also called community immunity) occurs when a sufficiently large proportion of a population becomes immune to a disease — through vaccination or prior infection — so the pathogen can no longer spread efficiently. • **Protecting the vulnerable** — Even unimmunised individuals (newborns, immunocompromised patients) gain indirect protection because the chain of transmission is broken when most people around them are immune. • The threshold percentage required for herd immunity varies by disease; measles requires about 95% immunity, while polio requires around 80-85%. • 💡 Option A (Immunity for one person) is wrong because individual immunity protects only that person, whereas herd immunity describes population-level protection; Option C (Immunity in animals only) is wrong because herd immunity applies to any population including humans; Option D (Immunity from eating meat) is wrong because dietary choices do not confer specific immune protection against infectious diseases.

Correct Answer: C. Eosinophils

• **Eosinophils** = granulocytes that specialise in attacking large multicellular parasites (helminths/worms) that are too big for regular phagocytosis; they release toxic enzymes and proteins directly onto the parasite's surface. • **Eosinophilia as a diagnostic sign** — A high eosinophil count (eosinophilia) in a blood test is a classic indicator of parasitic worm infection and helps doctors diagnose conditions like ascariasis or hookworm disease. • Eosinophils work in concert with IgE antibodies; IgE coats the parasite and eosinophils bind to the IgE, enhancing their cytotoxic attack. • 💡 Option A (B-cells) is wrong because B-cells produce antibodies but do not surge specifically in worm infections the way eosinophils do; Option B (Basophils) is wrong because basophils are involved in allergic reactions and release histamine, not the primary responders to worm infections; Option D (Neutrophils) is wrong because neutrophils are the first responders to bacterial infections via phagocytosis, not specialised for large parasites.

Correct Answer: A. T-lymphocytes

• **T-lymphocytes** = the key cells of cell-mediated immunity; they mature in the thymus and directly attack infected or abnormal cells rather than producing antibodies. • **Two main types of T-cells** — Cytotoxic T-cells (CD8+) kill virus-infected and tumour cells by injecting toxic proteins; Helper T-cells (CD4+) coordinate the entire immune response by releasing cytokines that activate both B-cells and other T-cells. • Cell-mediated immunity is especially critical against intracellular pathogens (viruses, some bacteria) that antibodies cannot reach once inside a cell. • 💡 Option B (B-lymphocytes) is wrong because B-lymphocytes drive humoral (antibody-mediated) immunity, not cell-mediated immunity; Option C (Skin) is wrong because skin is a physical/chemical barrier (innate immunity), not part of cell-mediated adaptive immunity; Option D (Antibodies) is wrong because antibodies are the products of B-cells and represent humoral immunity, the counterpart to cell-mediated immunity.

Correct Answer: B. To filter lymph and trap pathogens

• **To filter lymph and trap pathogens** = Lymph nodes are small, bean-shaped structures located along lymphatic vessels throughout the body; they act as biological filtration checkpoints where lymph fluid is screened for pathogens, debris, and cancer cells. • **Immune activation sites** — When pathogens are trapped in a lymph node, resident immune cells (macrophages, dendritic cells) present antigens to T-cells and B-cells, triggering an adaptive immune response; this is why lymph nodes swell during an infection. • There are about 600 lymph nodes in the human body, concentrated in the neck, armpits, and groin. • 💡 Option A (To digest food) is wrong because digestion occurs in the stomach and intestines, not lymph nodes; Option C (To pump blood) is wrong because pumping blood is the exclusive function of the heart; Option D (To produce hormones) is wrong because hormone production is the role of endocrine glands such as the thyroid and adrenal glands.

Correct Answer: C. IgE

• **IgE** = an antibody class that binds to allergens (pollen, dust mites, food proteins) or parasitic antigens and then cross-links receptors on mast cells and basophils, triggering the release of histamine and other inflammatory mediators. • **Dual role in allergy and anti-parasite defence** — Evolutionarily, IgE likely evolved to combat parasitic worm infections; in modern hygienic environments it often misfires against harmless allergens, producing conditions like hay fever and asthma. • Serum IgE levels are routinely measured in allergy testing; elevated IgE confirms allergic sensitisation. • 💡 Option A (IgM) is wrong because IgM is the first antibody produced in a primary immune response and is not specifically linked to allergies; Option B (IgG) is wrong because IgG is the main long-term protective antibody in blood and is associated with secondary immune responses, not allergic reactions; Option D (IgA) is wrong because IgA is the secretory antibody found on mucosal surfaces, not the allergy antibody.

Correct Answer: B. Innate Immunity

• **Innate Immunity** = the first layer of host defence that an individual is born with; it is non-specific, immediate (responds within minutes to hours), and does not require prior exposure to a pathogen. • **Components of innate immunity** — Includes physical barriers (skin, mucus), chemical barriers (stomach acid, lysozyme in tears), and cellular responses (phagocytes like neutrophils and macrophages, NK cells, and the complement system). • Unlike adaptive immunity, innate immunity has no immunological memory — it responds identically to the same pathogen on every encounter. • 💡 Option A (Active Immunity) is wrong because active immunity develops after a person is infected or vaccinated and involves B- and T-cell activation; Option C (Passive Immunity) is wrong because passive immunity is acquired by receiving ready-made antibodies from another source (e.g., maternal IgG via placenta); Option D (Adaptive Immunity) is wrong because adaptive immunity is acquired, specific, and develops over days to weeks after antigen exposure.