Immunity — Set 3

Correct Answer: D. Innate Immunity

• **Innate Immunity** = Innate immunity is the non-specific, first-line defence present from birth; it includes physical barriers (skin, mucous membranes), chemical defences (lysozyme, stomach acid), and cellular components (neutrophils, NK cells, macrophages) that respond immediately to any pathogen without prior exposure. • **Non-specific and no memory** — it treats all foreign invaders with the same general response and does not form immunological memory, unlike the adaptive immune system. • Innate immunity buys time for the slower but more precise adaptive immune response to mount. • 💡 Option A (Active Immunity) is wrong because active immunity requires the adaptive immune system to respond to a specific antigen and build memory — it develops over days and is not present from birth; Option B (Acquired Immunity) is wrong because acquired (adaptive) immunity is both specific and developed after exposure to an antigen; Option C (Passive Immunity) is wrong because passive immunity is obtained by receiving antibodies made by another organism — it is not present from birth as an inherent system.

Correct Answer: C. Bone Marrow

• **Bone Marrow** = Red bone marrow is the primary haematopoietic (blood-forming) tissue in adults; it produces all types of blood cells including all white blood cells (WBCs) — neutrophils, monocytes, eosinophils, basophils, and lymphocytes — through the process of haematopoiesis. • **Primary lymphoid organ** — bone marrow is also where B-lymphocytes mature fully, making it a primary lymphoid organ alongside the thymus. • In infants, haematopoiesis occurs in almost all bones, but in adults it is limited mainly to the sternum, vertebrae, ribs, pelvis, and the ends of long bones. • 💡 Option A (Kidney) is wrong because kidneys produce erythropoietin (a hormone that stimulates RBC production in bone marrow) but do not themselves produce blood cells; Option B (Stomach) is wrong because the stomach produces digestive acids and enzymes and has no blood cell-forming function; Option D (Heart) is wrong because the heart pumps blood but does not produce blood cells.

Correct Answer: B. Allergy

• **Allergy** = An allergy is a type I hypersensitivity reaction where the immune system overreacts to a normally harmless environmental substance (allergen) such as pollen, dust mites, or certain foods, treating it as a dangerous invader. • **IgE-mediated mechanism** — on first exposure, IgE antibodies are produced and attach to mast cells; on re-exposure, the allergen cross-links these IgE antibodies, triggering mast cells to release histamine and causing allergy symptoms (sneezing, itching, swelling). • Common allergens that trigger this exaggerated response include pollen, animal dander, latex, and certain foods like peanuts. • 💡 Option A (Autoimmunity) is wrong because autoimmunity is the immune system attacking the body's own cells, not reacting to external environmental substances; Option C (Vaccination) is wrong because vaccination is a deliberate intervention that builds protective immunity, not a harmful overreaction; Option D (Passive immunity) is wrong because passive immunity is the transfer of antibodies from an external source, a normal beneficial process, not an exaggerated harmful response.

Correct Answer: C. Helper T-cells

• **Helper T-cells** = Helper T-cells (CD4+ T-cells) are the master coordinators of adaptive immunity; after recognising an antigen presented by an APC, they secrete cytokines (e.g., IL-2, IL-4) that activate B-cells to produce antibodies and stimulate Cytotoxic T-cells to proliferate and kill infected cells. • **HIV destroys these cells** — HIV targets CD4+ Helper T-cells specifically, which is why HIV infection collapses both humoral (B-cell) and cell-mediated (Cytotoxic T-cell) branches of immunity simultaneously. • Without Helper T-cell signals, B-cells and Cytotoxic T-cells cannot be fully activated, making Helper T-cells indispensable for an effective adaptive immune response. • 💡 Option A (Platelets) is wrong because platelets are anucleate cell fragments involved in blood clotting, with no ability to secrete cytokines or coordinate immune responses; Option B (Red blood cells) is wrong because RBCs transport oxygen and carbon dioxide and have no immune regulatory role; Option D (Plasma cells) is wrong because plasma cells are the antibody-secreting end-product of B-cell activation — they are activated by Helper T-cells, not the ones that do the activating.

Correct Answer: D. Lymph

• **Lymph** = Lymph is the clear, slightly yellowish fluid that seeps out of blood capillaries into interstitial (tissue) spaces; lymphatic capillaries collect this fluid and carry it through lymph vessels to lymph nodes, where it is filtered by resident immune cells. • **Pathogen surveillance** — lymph nodes act as checkpoint stations; macrophages and dendritic cells within them trap pathogens, dead cells, and foreign particles from the lymph, preventing systemic spread of infection. • Filtered lymph eventually returns to the bloodstream via the thoracic duct, maintaining fluid balance between blood and tissues. • 💡 Option A (Urine) is wrong because urine is the waste filtrate produced by kidneys from blood — it is not collected and filtered by lymph nodes; Option B (Gastric juice) is wrong because gastric juice is the acidic digestive secretion of the stomach, confined to the gastrointestinal tract; Option C (Bile) is wrong because bile is produced by the liver and stored in the gall bladder for fat digestion — it travels through the bile duct into the intestine, not through lymph nodes.

Correct Answer: B. Skin

• **Skin** = The skin is the body's largest organ (~1.7–2.0 m²); its outermost layer (stratum corneum) consists of tightly packed, keratinised dead cells that form a tough, waterproof, physical barrier against the entry of bacteria, viruses, fungi, and other pathogens. • **Physical, not chemical or cellular** — unlike antibodies or interferons, the skin works purely by mechanical exclusion: it blocks pathogens rather than responding to them biochemically. • The slightly acidic surface pH (~4.5–5.5) of healthy skin also inhibits many bacterial species, adding a mild chemical dimension to this physical barrier. • 💡 Option A (Interferon) is wrong because interferons are proteins secreted by virus-infected cells — they are a chemical/cellular response, not a physical structure; Option C (Complement protein) is wrong because complement proteins are soluble molecules in blood plasma that lyse pathogens through a biochemical cascade — not a physical barrier; Option D (Antibody) is wrong because antibodies are specific protein molecules produced by B-cells in response to antigens — they belong to the adaptive immune system, not the physical barrier category.

Correct Answer: C. Specificity

• **Specificity** = Specificity is the property of the adaptive immune system that allows it to recognise and respond to one particular antigen (or even a single epitope on an antigen) with extreme precision — each B-cell receptor or T-cell receptor is complementary to only one specific molecular shape. • **Basis of targeted immunity** — specificity ensures that the antibodies and T-cells produced against, say, the influenza virus will not cross-react with the measles virus, enabling targeted clearance of a particular pathogen. • This property, along with memory, distinguishes adaptive immunity from the broad-spectrum innate immune response. • 💡 Option A (Mobility) is wrong because while immune cells are mobile (they travel through blood and lymph), mobility is a mechanical property, not the defining characteristic of antigen recognition; Option B (Toxicity) is wrong because toxicity describes the ability to cause harm — while some immune responses damage tissue, toxicity is not the term for pathogen recognition; Option D (Solubility) is wrong because solubility is a physical chemistry property describing whether a substance dissolves in water — completely unrelated to immune recognition.

Correct Answer: C. Neutrophils

• **Neutrophils** = Neutrophils make up 60–70% of all white blood cells in normal human blood; they are the first responders to infection, arriving at the site within minutes by following chemical signals (chemokines) released by damaged tissues and pathogens. • **Rapid phagocytic killers** — neutrophils engulf pathogens via phagocytosis, release toxic chemicals (reactive oxygen species, lysozyme) from granules, and can also cast neutrophil extracellular traps (NETs) to ensnare bacteria. • They are short-lived cells (survive only a few hours to days at infection sites), and dead neutrophils form the pus seen in infected wounds. • 💡 Option A (Lymphocytes) is wrong because lymphocytes (B- and T-cells) are part of the adaptive immune response — they arrive later and require activation by antigen-presenting cells before responding; Option B (Basophils) is wrong because basophils are rare WBCs involved in allergic reactions and anti-parasitic responses, not rapid first-response phagocytosis; Option D (Monocytes) is wrong because monocytes are phagocytic but circulate in blood and migrate into tissues more slowly, maturing into macrophages — they are not the first to arrive at infection sites.

Correct Answer: A. Antivenom

• **Antivenom** = Antivenom (also called antivenin) is produced by injecting small doses of snake venom into horses or sheep, collecting the antibodies they produce, and purifying those antibodies; when injected into a snakebite victim, these pre-formed antibodies immediately neutralise the circulating venom toxins. • **Artificial passive immunity** — antivenom is the classic example of artificial passive immunity: antibodies are delivered externally for instant protection, with no time for the victim's own immune system to act. • Speed is critical — antivenom is most effective within the first few hours of a bite before venom causes irreversible organ damage. • 💡 Option B (Vitamin) is wrong because vitamins are micronutrients that support metabolic functions and cannot neutralise toxins; Option C (Antibiotic) is wrong because antibiotics kill bacteria by disrupting their cell walls or protein synthesis — they have no effect on venom, which is a protein toxin, not a living organism; Option D (Vaccine) is wrong because a vaccine builds future immunity by stimulating memory cell production — it requires days to weeks to work and cannot help in an acute emergency.

Correct Answer: B. Antibodies

• **Antibodies** = Humoral immunity (from the Latin 'humor' meaning fluid) refers to the branch of adaptive immunity mediated by antibodies circulating in blood and lymph; B-cells produce these antibodies, which neutralise pathogens and toxins present in body fluids outside cells. • **Contrast with cell-mediated immunity** — while humoral immunity combats extracellular threats via antibodies, cell-mediated immunity (driven by T-cells) targets intracellular pathogens like viruses inside host cells; both branches are necessary for complete protection. • The term 'humoral' was coined historically when scientists discovered that serum (the fluid component of blood) could transfer immunity between animals. • 💡 Option A (T-cells) is wrong because T-cells are the primary mediators of cell-mediated immunity, not humoral immunity — though Helper T-cells do assist B-cells in producing antibodies; Option C (Skin) is wrong because the skin is a physical barrier component of innate immunity, not a participant in antibody-mediated humoral responses; Option D (Platelets) is wrong because platelets are anucleate fragments involved in blood clotting (haemostasis) and have no role in antibody production or humoral immunity.