Immunity — Set 1

Correct Answer: A. B-lymphocytes

• **B-lymphocytes** = B-lymphocytes are the only white blood cells that differentiate into plasma cells, which then secrete millions of antigen-specific antibodies into the bloodstream. • **Humoral arm of immunity** — this antibody-secreting role makes B-lymphocytes the cornerstone of humoral (antibody-mediated) immunity, distinct from cell-mediated immunity. • When an antigen is encountered, B-cells are activated (often with Helper T-cell help), undergo clonal expansion, and some become long-lived memory B-cells. • 💡 Option B (Monocytes) is wrong because monocytes are phagocytes that engulf debris, not antibody producers; Option C (Eosinophils) is wrong because eosinophils combat parasites and mediate allergic inflammation, not antibody synthesis; Option D (T-lymphocytes) is wrong because T-cells mediate cell-killing and cytokine signaling, not direct antibody production.

Correct Answer: A. To remember previous infections

• **To remember previous infections** = Memory cells are long-lived B- or T-lymphocytes formed after a primary immune response; they persist in the body for years and recognise the same antigen on re-exposure. • **Basis of immunological memory** — upon second encounter with the pathogen, memory cells trigger a faster, stronger secondary response, often clearing the infection before symptoms appear. • This is why a person who recovers from chickenpox rarely gets it again, and it is also the scientific principle behind vaccination. • 💡 Option B (To engulf pathogens) is wrong because phagocytosis is done by neutrophils and macrophages, not memory cells; Option C (To produce toxins) is wrong because toxin production is a bacterial strategy, not an immune-cell function; Option D (To clot blood) is wrong because clotting is performed by platelets and clotting factors, entirely unrelated to lymphocytes.

Correct Answer: A. Skin and Mucous membranes

• **Skin and Mucous membranes** = Skin forms an unbroken physical barrier of keratinised cells that most pathogens cannot penetrate, while mucous membranes lining airways and the gut trap microbes in sticky mucus. • **Chemical reinforcement** — skin secretes sebum (slightly acidic) and mucous membranes produce lysozyme and IgA antibodies, adding a chemical layer on top of the physical one. • Being present at birth and requiring no prior exposure, these barriers are classified as innate (non-specific) immunity — the very first line of defence. • 💡 Option B (Antibodies) is wrong because antibodies are part of the specific adaptive immune response, which acts only after a pathogen has already entered; Option C (Inflammation) is wrong because inflammation is a secondary innate response that occurs after tissue is breached; Option D (Interferons) is wrong because interferons are signalling proteins released only after a cell is already infected by a virus.

Correct Answer: D. Spleen

• **Spleen** = The spleen is the largest secondary lymphoid organ; it filters blood continuously, breaking down aged and damaged red blood cells (RBCs) and recycling haemoglobin — hence the nickname 'graveyard of RBCs'. • **Dual immune role** — it also houses large numbers of lymphocytes and macrophages that detect and destroy blood-borne pathogens, making it a key immunological filter. • The iron recovered from broken-down haemoglobin is sent to the liver for reuse in new RBC production. • 💡 Option A (Liver) is wrong because the liver processes toxins and bile but is not the primary site for RBC destruction; Option B (Kidney) is wrong because kidneys filter blood to form urine, not to destroy old blood cells; Option C (Pancreas) is wrong because the pancreas produces digestive enzymes and hormones like insulin, with no role in RBC destruction.

Correct Answer: D. Natural Active Immunity

• **Natural Active Immunity** = When a person actually contracts chickenpox, their immune system mounts a response — producing antibodies and memory cells naturally, without any external intervention. • **Long-lasting protection** — the memory cells formed persist for decades, granting lifelong or near-lifelong protection against the same pathogen on subsequent exposure. • The word 'natural' indicates no artificial injection was involved, and 'active' indicates the person's own immune system did the work. • 💡 Option A (Innate Immunity) is wrong because innate immunity is non-specific and does not create lasting memory; Option B (Artificial Passive Immunity) is wrong because that refers to injecting someone else's ready-made antibodies (like antivenom); Option C (Natural Passive Immunity) is wrong because that is the transfer of maternal antibodies to a baby through breast milk, not the recipient's own immune response.

Correct Answer: A. Histamine

• **Histamine** = Mast cells store pre-formed histamine in granules; when an allergen cross-links IgE antibodies on the mast cell surface, the cell degranulates and releases histamine rapidly. • **Vasodilation and classic allergy symptoms** — histamine dilates blood vessels and increases their permeability, causing the redness, swelling, itching, and runny nose associated with allergic reactions. • Antihistamine drugs (like cetirizine) work by blocking histamine receptors, which is why they relieve allergy symptoms. • 💡 Option B (Hemoglobin) is wrong because haemoglobin is the oxygen-carrying protein inside red blood cells and plays no role in allergic signalling; Option C (Insulin) is wrong because insulin is a pancreatic hormone that regulates blood glucose; Option D (Thyroxine) is wrong because thyroxine is a thyroid hormone controlling metabolic rate, not allergy.

Correct Answer: D. Natural Passive Immunity

• **Natural Passive Immunity** = In natural passive immunity, ready-made antibodies are transferred from mother to infant — through the placenta (IgG) before birth and through colostrum/breast milk (IgA) after birth — without any immune effort by the baby. • **Temporary protection** — because the baby's own immune system did not produce these antibodies, it forms no memory cells, so protection fades within a few months. • This bridge immunity is critical while the infant's own adaptive immune system matures. • 💡 Option A (Artificial Passive Immunity) is wrong because that involves injecting antibody-containing serum (e.g., antivenom or immunoglobulin shots) by medical intervention, not a natural mother–child transfer; Option B (Innate Immunity) is wrong because innate immunity is non-specific and present from birth, not transferred antibodies; Option C (Active Immunity) is wrong because active immunity requires the recipient's own immune cells to respond and build memory.

Correct Answer: A. Thymus

• **Thymus** = The thymus is a bi-lobed primary lymphoid organ situated in the upper chest (mediastinum); immature T-cells (thymocytes) migrate here from bone marrow and undergo maturation and selection. • **T-cell education** — the thymus 'educates' T-cells to distinguish self from non-self and eliminates those that would attack the body's own tissues (a process called negative selection). • The thymus is most active during childhood and gradually shrinks (involutes) after puberty, yet memory T-cells formed earlier persist lifelong. • 💡 Option B (Thyroid) is wrong because the thyroid gland secretes thyroxine (T3/T4) and calcitonin for metabolic regulation, not T-cell maturation; Option C (Adrenal) is wrong because the adrenal glands produce adrenaline and cortisol for stress response; Option D (Pituitary) is wrong because the pituitary is the master endocrine gland secreting growth hormone, TSH, etc., with no role in T-cell maturation.

Correct Answer: C. Autoimmunity

• **Autoimmunity** = In autoimmunity, the immune system loses self-tolerance — it mistakenly identifies the body's own healthy cells or proteins as foreign antigens and mounts destructive immune attacks against them. • **Examples of autoimmune diseases** — Rheumatoid arthritis (attacks joints), Type 1 Diabetes (attacks pancreatic beta cells), and Multiple Sclerosis (attacks myelin sheaths of nerves) are all autoimmune disorders. • The exact cause is not always known, but genetic predisposition and certain infections are considered triggers. • 💡 Option A (Hypersensitivity) is wrong because hypersensitivity is an exaggerated reaction to an external antigen (e.g., allergen), not an attack on self-tissues; Option B (Immunodeficiency) is wrong because immunodeficiency means the immune system is too weak to fight off infections, the opposite of autoimmunity; Option D (Acquired immunity) is wrong because acquired immunity is the normal adaptive immune response to external pathogens, not a malfunction.

Correct Answer: D. Interferons

• **Interferons** = When a cell is infected by a virus, it produces and secretes interferon proteins as a warning signal to neighbouring healthy cells, prompting them to activate antiviral defences before the virus can spread. • **Mechanism of action** — interferons induce neighbouring cells to produce enzymes that degrade viral RNA and inhibit viral protein synthesis, effectively creating an antiviral state in surrounding tissue. • Interferons also activate natural killer (NK) cells and macrophages, bridging innate and adaptive immunity. • 💡 Option A (Globulins) is wrong because globulins are blood proteins that include antibodies and carrier proteins, not the antiviral signalling molecules released by infected cells; Option B (Albumins) is wrong because albumin is a blood protein that maintains osmotic pressure, with no antiviral signalling role; Option C (Antigens) is wrong because antigens are surface molecules on pathogens that trigger immune responses — they are not produced by the host cell to protect neighbours.