Immunity — Set 5

Correct Answer: B. Polio

• **Polio** = a highly contagious viral disease caused by the poliovirus that attacks the nervous system and can cause irreversible paralysis within hours; it primarily affects children under 5. • **Near-eradication through vaccination** — Widespread use of the oral polio vaccine (OPV) and inactivated polio vaccine (IPV) has reduced global polio cases by over 99% since 1988; India was declared polio-free in 2014. • Polio has no cure once contracted, making vaccination the only effective preventive strategy. • 💡 Option A (Diabetes) is wrong because diabetes is a metabolic disorder caused by insulin dysfunction, not an infectious pathogen, so a vaccine cannot prevent it; Option C (Asthma) is wrong because asthma is an allergic/inflammatory condition of the airways, not caused by a specific infectious agent; Option D (Scurvy) is wrong because scurvy results from Vitamin C deficiency, not an infection, and is prevented through diet, not vaccines.

Correct Answer: B. They help antibodies destroy bacteria

• **They help antibodies destroy bacteria** = The complement system is a cascade of ~30 plasma proteins that 'complement' (enhance) the ability of antibodies and phagocytes to clear pathogens from the body. • **Three key mechanisms** — Complement proteins can (1) directly lyse bacteria by forming a membrane attack complex (MAC) that punches holes in the bacterial cell wall, (2) opsonise pathogens (coat them) to make them more attractive to phagocytes, and (3) attract more immune cells to the infection site through chemotaxis. • The complement system bridges innate and adaptive immunity and can be activated even in the absence of antibodies. • 💡 Option A (They carry oxygen) is wrong because oxygen is transported by haemoglobin inside red blood cells, not by complement proteins; Option C (They produce vitamins) is wrong because vitamins are obtained from diet or synthesised in specific tissues (e.g., Vitamin D in skin), not by immune proteins; Option D (They strengthen bones) is wrong because bone strength depends on calcium, phosphorus, and osteoblast activity, entirely unrelated to the complement system.

Correct Answer: A. Bone Marrow

• **Bone Marrow** = the spongy tissue inside bones (especially the femur, sternum, and pelvis) that contains haematopoietic stem cells — multipotent cells capable of giving rise to all types of blood cells. • **All lineages originate here** — Red blood cells (erythrocytes), platelets (thrombocytes), and all white blood cells (granulocytes, monocytes, B-lymphocytes) are born in the bone marrow; T-lymphocytes are also born here but mature in the thymus. • In adults, active red bone marrow is found mainly in flat bones; conditions like leukaemia directly affect bone marrow function. • 💡 Option B (Spleen) is wrong because the spleen filters blood and hosts immune cells but does not produce new blood cells in adults; Option C (Heart) is wrong because the heart is a muscular pump and has no role in blood cell production; Option D (Liver) is wrong because the liver produces blood cells only in the foetal stage (extramedullary haematopoiesis), not in adults.

Correct Answer: B. IgM

• **IgM** = the first antibody class secreted by newly activated B-cells during a primary (first-time) immune response; it appears in the blood within days of antigen exposure, providing early defence while other antibodies are still being developed. • **Pentameric structure for high avidity** — IgM exists as a pentamer (5 antibody units joined together), giving it 10 antigen-binding sites; this large size makes it exceptionally efficient at agglutinating (clumping) pathogens and activating the complement system. • During a secondary response (re-exposure), the immune system 'class switches' and IgG becomes the dominant antibody, which is why IgM levels indicate a current or very recent infection. • 💡 Option A (IgA) is wrong because IgA is the secretory antibody on mucosal surfaces and is not the first produced in blood during a primary response; Option C (IgE) is wrong because IgE is specifically induced by allergens and parasitic antigens, not first in a general primary immune response; Option D (IgG) is wrong because IgG rises later in a primary response and dominates during the secondary (memory) response.

Correct Answer: B. Parietal cells

• **Parietal cells** = oxyntic cells located in the gastric glands of the stomach lining that secrete hydrochloric acid (HCl) and intrinsic factor; the HCl creates a highly acidic environment (pH 1.5–2) that denatures proteins and kills most ingested bacteria and viruses. • **First chemical barrier of innate immunity** — Stomach acid is a critical non-specific immune barrier; pathogens like Salmonella and most food-borne viruses are destroyed before they can reach the intestines. • The same HCl that protects against infection also activates pepsinogen into pepsin for protein digestion, making parietal cells dual-function cells. • 💡 Option A (Alpha cells) is wrong because alpha cells are in the pancreatic islets of Langerhans and secrete glucagon to raise blood sugar; Option C (Beta cells) is wrong because beta cells are also pancreatic and secrete insulin to lower blood sugar; Option D (Goblet cells) is wrong because goblet cells secrete mucus to protect the lining of the gut and respiratory tract, not acid.

Correct Answer: A. Killing virus-infected and tumor cells

• **Killing virus-infected and tumor cells** = Natural Killer cells are large granular lymphocytes of the innate immune system that patrol the body and destroy cells displaying abnormal surface markers — specifically cells that have lost MHC class I molecules (a hallmark of viral infection or cancerous transformation). • **No prior sensitisation required** — Unlike cytotoxic T-cells, NK cells do not need prior exposure or antigen presentation; they respond rapidly (within hours) making them vital for early containment of viral infections and tumour surveillance. • NK cells kill target cells by releasing perforins (which punch holes in the cell membrane) and granzymes (which trigger apoptosis inside the target cell). • 💡 Option B (Producing antibodies) is wrong because antibody production is the exclusive role of B-lymphocytes (plasma cells); NK cells have no antibody-secreting capacity; Option C (Carrying oxygen) is wrong because oxygen transport is performed by haemoglobin in red blood cells; Option D (Clotting blood) is wrong because blood clotting is carried out by platelets and clotting factors, not lymphocytes.

Correct Answer: C. It has memory

• **It has memory** = The defining feature of acquired (adaptive) immunity is immunological memory — after the first encounter with a pathogen, long-lived memory B-cells and T-cells are formed that persist for years or even a lifetime. • **Faster and stronger secondary response** — On re-exposure to the same pathogen, memory cells enable a response that is much faster, larger, and more effective than the primary response, often eliminating the pathogen before symptoms appear; this is the biological basis of how vaccines work. • Adaptive immunity is also highly specific — each B-cell and T-cell clone responds to a single antigen, unlike the non-specific innate system. • 💡 Option A (It is present from birth) is wrong because innate immunity is present from birth; adaptive immunity must be developed through antigen exposure; Option B (It is very fast) is wrong because innate immunity responds within minutes-hours, while adaptive immunity takes 5–7 days for a primary response; Option D (It is non-specific) is wrong because innate immunity is non-specific; adaptive immunity is the opposite — it is highly specific to a particular antigen.

Correct Answer: D. Spleen

• **Spleen** = the largest lymphoid organ in the human body, located in the upper left quadrant of the abdomen behind the stomach; it serves as a blood filter and immune organ simultaneously. • **Dual immune functions** — The spleen's white pulp contains lymphocytes that respond to blood-borne antigens, while its red pulp filters old and damaged red blood cells and stores monocytes that can be rapidly deployed to sites of infection or injury. • The spleen is also important in immune responses against encapsulated bacteria (like Streptococcus pneumoniae); people who have had their spleen removed are at increased risk from such infections. • 💡 Option A (Thymus) is wrong because the thymus is located in the upper chest (mediastinum) behind the sternum, not the abdomen; Option B (Appendix) is wrong because the appendix is a small pouch attached to the large intestine in the lower right abdomen; Option C (Pancreas) is wrong because the pancreas is an endocrine/exocrine gland located behind the stomach but is not a lymphoid organ.

Correct Answer: B. Swelling and Redness

• **Swelling and Redness** = the classic signs of inflammation (along with heat and pain, together called the 'cardinal signs'); redness (rubor) results from vasodilation increasing blood flow to the affected tissue, while swelling (tumor) results from increased vascular permeability allowing fluid and immune cells to leak into the tissue. • **Protective purpose** — The inflammatory response is a beneficial innate immune mechanism designed to contain infection, recruit immune cells to the site, and begin tissue repair; the increased blood flow brings antibodies, phagocytes, and nutrients necessary for healing. • Inflammation is triggered by chemical signals (histamine, prostaglandins, cytokines) released by damaged cells and mast cells at the injury site. • 💡 Option A (Muscle growth) is wrong because muscle growth (hypertrophy) results from exercise and protein synthesis, not from immune inflammation; Option C (Lower body temperature) is wrong because inflammation typically raises local and systemic temperature (fever), not lowers it; Option D (Decreased heart rate) is wrong because inflammation and the associated stress response typically increase heart rate to supply more blood to the affected area.

Correct Answer: C. Activating other immune cells

• **Activating other immune cells** = Helper T-cells (CD4+ T-cells) are the master coordinators of the adaptive immune response; they do not directly kill pathogens but instead release cytokines (chemical signals like interleukins and interferons) that activate and amplify the responses of B-cells, cytotoxic T-cells, and macrophages. • **Central role in immunity** — Without Helper T-cells, the adaptive immune system cannot mount an effective response; this is precisely why HIV is so deadly — it destroys CD4+ Helper T-cells, collapsing the entire coordinated immune defence. • Helper T-cells become activated when they recognise a pathogen fragment presented by antigen-presenting cells (dendritic cells, macrophages) via MHC class II molecules. • 💡 Option A (Storing energy) is wrong because energy is stored in fat cells (adipocytes) and liver cells as glycogen, not in immune T-cells; Option B (Producing stomach acid) is wrong because stomach acid is produced by parietal cells of the gastric lining, not by lymphocytes; Option D (Destroying bacteria directly) is wrong because direct killing of infected cells is the role of cytotoxic T-cells (CD8+), not helper T-cells.