Immunity — Set 2

Correct Answer: D. Inflammation

• **Inflammation** = Inflammation is the body's rapid, non-specific innate response to tissue damage or infection; its cardinal signs are redness (rubor), heat (calor), swelling (tumor), pain (dolor), and loss of function. • **Why these signs occur** — damaged cells release chemical mediators (histamine, prostaglandins) that dilate local blood vessels (redness, heat) and increase vascular permeability (swelling), while nerve endings are stimulated (pain). • Inflammation also recruits phagocytes to the site to destroy pathogens, making it a protective—though uncomfortable—response. • 💡 Option A (Neutralization) is wrong because neutralization is a specific antibody–antigen reaction in which antibodies block pathogen activity — it is part of adaptive immunity, not a non-specific tissue response; Option B (Vaccination) is wrong because vaccination is a deliberate introduction of antigens to build immunological memory; Option C (Agglutination) is wrong because agglutination is the clumping of pathogens by antibodies, a specific adaptive immune mechanism.

Correct Answer: D. Macrophages

• **Macrophages** = Macrophages are professional antigen-presenting cells (APCs); they engulf and digest pathogens, then display pathogen-derived peptide fragments on MHC class II molecules on their surface for recognition by Helper T-cells. • **Bridge between innate and adaptive immunity** — by presenting antigens to T-cells, macrophages activate the adaptive immune system and are therefore essential for launching a targeted, specific immune response. • Dendritic cells and B-cells are also APCs, but macrophages are the most commonly cited example in exam contexts. • 💡 Option A (Platelets) is wrong because platelets are cell fragments involved in blood clotting, not antigen presentation; Option B (Red blood cells) is wrong because RBCs have no nucleus, carry oxygen, and lack the MHC class II molecules needed for antigen presentation; Option C (Plasma cells) is wrong because plasma cells are terminally differentiated B-cells that secrete antibodies — they present antigens passively but are not classified as professional APCs.

Correct Answer: A. Antibodies

• **Antibodies** = Antibodies (immunoglobulins) are Y-shaped glycoproteins produced by plasma cells; each arm of the Y carries an antigen-binding site (paratope) that fits precisely onto a specific part of an antigen (epitope), like a lock and key. • **Effector functions** — bound antibodies neutralise pathogens, flag them for phagocytosis (opsonisation), activate the complement system, or trigger antibody-dependent cell-mediated cytotoxicity (ADCC). • There are five classes: IgG, IgA, IgM, IgD, and IgE — each with distinct roles in different immune compartments. • 💡 Option B (Vitamins) is wrong because vitamins are organic micronutrients required for metabolic reactions, with no antigen-binding function; Option C (Complement proteins) is wrong because complement proteins are a cascade of enzymes in blood plasma that lyse pathogens — they are not Y-shaped, do not bind specific antigens with high specificity, and are not secreted by plasma cells; Option D (Toxins) is wrong because toxins are harmful substances produced by pathogens, the very things antibodies defend against.

Correct Answer: C. AIDS

• **AIDS** = Acquired Immunodeficiency Syndrome (AIDS) is caused by the Human Immunodeficiency Virus (HIV), which specifically infects and destroys CD4+ Helper T-cells, gradually dismantling the entire adaptive immune system. • **Progression and consequence** — as Helper T-cell counts fall below 200 cells/µL, the body becomes vulnerable to opportunistic infections (like Pneumocystis pneumonia) and certain cancers that a healthy immune system would normally suppress. • AIDS is classified as an acquired immunodeficiency — the immune system was functional at birth but became deficient due to an external infection. • 💡 Option A (Asthma) is wrong because asthma is a chronic inflammatory airway condition triggered by allergens — it is a hypersensitivity disorder, not immunodeficiency; Option B (Psoriasis) is wrong because psoriasis is an autoimmune skin disorder where the immune system attacks skin cells; Option D (Lupus) is wrong because Systemic Lupus Erythematosus is an autoimmune condition in which the body attacks its own tissues — the opposite of immunodeficiency.

Correct Answer: D. Killing infected or cancerous cells

• **Killing infected or cancerous cells** = Cytotoxic T-cells (CD8+ T-cells) scan the body for cells displaying foreign peptides on MHC class I molecules; when found, they induce the target cell to undergo apoptosis (programmed self-destruction), eliminating the threat. • **Mechanism of killing** — Cytotoxic T-cells release perforin (which punches pores in the target cell membrane) and granzymes (proteases that trigger the apoptotic cascade), ensuring precise destruction. • This cell-mediated killing is essential for clearing intracellular pathogens (viruses, some bacteria) and mutated cancerous cells that antibodies cannot reach inside a cell. • 💡 Option A (Transporting oxygen) is wrong because oxygen transport is the exclusive job of haemoglobin inside red blood cells; Option B (Storing minerals) is wrong because mineral storage (e.g., calcium, phosphorus) is done by bones; Option C (Producing mucus) is wrong because mucus is secreted by goblet cells lining respiratory and gastrointestinal tracts — a physical barrier function, not a T-cell function.

Correct Answer: D. Lysozyme

• **Lysozyme** = Lysozyme is a hydrolytic enzyme found in tears, saliva, nasal secretions, and breast milk; it destroys bacteria by cleaving the glycosidic bonds in peptidoglycan — the structural polymer that forms bacterial cell walls. • **Part of first-line defence** — because lysozyme works non-specifically on a wide range of bacteria without prior exposure, it is considered a chemical component of the innate (non-specific) first line of defence. • Alexander Fleming discovered lysozyme in 1921, over a decade before his discovery of penicillin. • 💡 Option A (Amylase) is wrong because amylase is a digestive enzyme that breaks down starch into sugars — it has no antibacterial activity; Option B (Lipase) is wrong because lipase digests dietary fats and has no role in killing bacteria; Option C (Pepsin) is wrong because pepsin is a protease that digests proteins in the stomach's acidic environment and plays no direct antibacterial role in tears or saliva.

Correct Answer: D. Does not involve the body's own cells

• **Does not involve the body's own cells** = In passive immunity, the recipient is given ready-made antibodies produced externally — either by another person, an animal, or in a laboratory — so the recipient's own B-cells and T-cells are never activated. • **No immunological memory formed** — since the body's own immune cells do not participate, no memory cells are created, and protection lasts only as long as the transferred antibodies survive (typically weeks to a few months). • This is the defining contrast with active immunity, where the body's own immune machinery is engaged and long-lasting memory is built. • 💡 Option A (Is very slow) is wrong because passive immunity actually provides immediate protection — there is no wait for an immune response to build, which is why antivenom is used for emergencies; Option B (Produces memory cells) is wrong because the absence of memory cell formation is precisely the hallmark of passive immunity; Option C (Lasts forever) is wrong because passive immunity is short-lived since the donor antibodies eventually degrade and are not replenished.

Correct Answer: A. Artificial Active Immunity

• **Artificial Active Immunity** = Vaccination introduces a harmless form of a pathogen (killed, attenuated, or its antigen fragments) into the body artificially; the immune system responds actively by producing antibodies and memory cells — without the person ever suffering the actual disease. • **Long-lasting memory without illness** — because the person's own immune cells do the work, memory cells are formed that provide durable (often lifelong) protection, just as with natural active immunity but without the risk of disease. • The word 'artificial' distinguishes it from natural active immunity (getting the real disease); the word 'active' distinguishes it from passive immunity (receiving pre-made antibodies). • 💡 Option B (Blood transfusion) is wrong because blood transfusion transfers red blood cells for medical treatment, not antigens for immune priming; Option C (Treatment after infection) is wrong because vaccination is a prophylactic (preventive) measure given before exposure, not a cure after infection; Option D (Natural Passive Immunity) is wrong because that refers to antibodies passed naturally from mother to child, with no deliberate medical intervention.

Correct Answer: D. It is engulfed and digested

• **It is engulfed and digested** = During phagocytosis, the phagocyte (neutrophil or macrophage) extends pseudopodia around the pathogen, engulfing it into an internal vesicle called a phagosome; the phagosome then fuses with a lysosome to form a phagolysosome, where powerful enzymes and reactive oxygen species digest and destroy the pathogen. • **Antigen presentation follows** — macrophages display fragments of the digested pathogen on MHC class II molecules, linking phagocytosis to adaptive immunity activation. • Phagocytosis is a key mechanism of innate immunity, operating within minutes of infection without requiring prior exposure. • 💡 Option A (It is released into blood) is wrong because the pathogen is destroyed inside the cell, not released — releasing it would spread infection further; Option B (It is turned into a vitamin) is wrong because no biological process converts pathogens into vitamins; Option C (It is cloned) is wrong because cloning the pathogen would amplify infection — the immune system does the opposite, producing more immune cells (clonal expansion of lymphocytes), not pathogen copies.

Correct Answer: B. IgG

• **IgG** = IgG constitutes approximately 75–80% of all circulating antibodies in adult human blood, making it by far the most abundant immunoglobulin; it is the main antibody of the secondary (memory) immune response. • **Versatile protector** — IgG crosses the placenta to confer passive immunity to the foetus, neutralises toxins and viruses, opsonises bacteria for phagocytosis, and activates the complement system. • Its long half-life of about 21 days (the longest of all Ig classes) contributes to its dominance in circulation. • 💡 Option A (IgE) is wrong because IgE is the least abundant immunoglobulin in blood (trace amounts) and is associated with allergic reactions and anti-parasitic responses; Option C (IgM) is wrong because IgM is the first antibody produced in a primary response — it is large (pentameric) and confined mainly to the bloodstream, but constitutes far less of total Ig than IgG; Option D (IgA) is wrong because IgA is the most abundant antibody in secretions (saliva, tears, breast milk), not in the bloodstream.