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Question 1. Which of the following structures provides the first physical barrier against most microbial invasion? A) Mucous membranes B) Skin keratinized layer C) Ciliated epithelium D) Gastric acid Answer: B Explanation: The keratinized epidermis of the skin is the outermost physical barrier that prevents entry of most pathogens. Question 2. Which innate cell type is primarily responsible for recognizing virus‑infected cells through the detection of reduced MHC I expression? A) Neutrophil B) Macrophage C) Natural Killer (NK) cell D) Dendritic cell Answer: C Explanation: NK cells detect cells with down‑regulated MHC I, a common viral evasion strategy, and induce cytotoxicity. Question 3. The process by which neutrophils exit the bloodstream and migrate toward sites of infection is called: A) Diapedesis B) Chemotaxis C) Phagocytosis D) Opsonization Answer: B
Explanation: Chemotaxis is the directed movement of cells toward higher concentrations of chemotactic factors. Question 4. Which cytokine is most important for the acute phase inflammatory response and induces fever? A) IL‑ 10 B) IFN‑γ C) IL‑ 1 β D) TGF‑β Answer: C Explanation: IL‑ 1 β acts on the hypothalamus to raise body temperature and stimulates acute‑phase protein synthesis. Question 5. Complement component C3b functions mainly as: A) A membrane attack complex subunit B) An opsonin that enhances phagocytosis C) A chemotactic factor for neutrophils D) An anaphylatoxin causing vasodilation Answer: B Explanation: C3b covalently binds to microbial surfaces, tagging them for phagocytosis. Question 6. Which of the following is NOT a characteristic of adaptive immunity? A) Antigen specificity B) Memory formation C) Rapid response within minutes of infection D) Clonal selection of lymphocytes
D) They release histamine during allergic reactions. Answer: C Explanation: Dendritic cells are potent APCs that process antigen and travel to lymphoid tissue to activate naïve T cells. Question 10. During inflammation, increased capillary permeability primarily results from the action of: A) Histamine and bradykinin B) Interferon‑α C) IL‑ 12 D) TNF‑β Answer: A Explanation: Histamine and bradykinin cause endothelial gaps, allowing plasma proteins and leukocytes to extravasate. Question 11. Which immunoglobulin is most abundant in serum and provides the majority of secondary immune response? A) IgM B) IgG C) IgA D) IgE Answer: B Explanation: IgG is the predominant serum antibody and is highly efficient in opsonization and neutralization during secondary responses. Question 12. The first antibody produced in a primary immune response is: A) IgG
B) IgA C) IgM D) IgE Answer: C Explanation: IgM is the initial class secreted by activated B cells before class switching occurs. Question 13. Which immunoglobulin is the primary antibody found in mucosal secretions? A) IgM B) IgG C) IgA D) IgD Answer: C Explanation: Dimeric secretory IgA is transported across mucosal epithelium and protects against pathogens at body surfaces. Question 14. The process by which B cells change antibody isotype from IgM to IgG, IgA, or IgE is called: A) Somatic hypermutation B) Class switch recombination C) Affinity maturation D) V(D)J recombination Answer: B Explanation: Class switch recombination alters the constant region of the heavy chain, changing antibody effector function.
Explanation: IFN‑γ produced by Th1 cells enhances macrophage microbicidal activity. Question 18. Which of the following best describes opsonization? A) Direct lysis of bacteria by antibodies B) Tagging of pathogens for enhanced phagocytosis C) Production of cytokines by T cells D) Activation of the complement cascade Answer: B Explanation: Opsonins (e.g., IgG, C3b) coat microbes, facilitating recognition and ingestion by phagocytes. Question 19. In a secondary immune response, the latency period before antibody production is typically: A) 1–2 days B) 5–7 days C) 10–14 days D) 21 days Answer: A Explanation: Memory B cells respond rapidly, producing high‑affinity antibodies within 1–2 days. Question 20. Which of the following immunoglobulins is most closely associated with allergic (type I hypersensitivity) reactions? A) IgM B) IgG C) IgA D) IgE
Answer: D Explanation: IgE binds FcεRI on mast cells and basophils; cross‑linking triggers degranulation in allergies. Question 21. The ELISA format that detects patient antibodies against a known antigen is: A) Direct ELISA B) Indirect ELISA C) Sandwich ELISA D) Competitive ELISA Answer: B Explanation: In indirect ELISA, antigen is coated, patient serum is added, and bound antibodies are detected with labeled secondary antibody. Question 22. In a sandwich ELISA, the capture antibody is immobilized on the plate. What is the purpose of the detection antibody? A) To block nonspecific binding B) To bind a different epitope on the antigen for signal generation C) To degrade excess antigen D) To change the pH of the reaction Answer: B Explanation: The detection antibody binds a second epitope, allowing enzymatic labeling for quantitation. Question 23. Which serologic test is most appropriate for confirming a recent infection with a virus that produces a short‑lived IgM response? A) Western blot for IgG B) IgM‑capture ELISA
C) Pacific Blue D) Alexa 647 Answer: A Explanation: PE has an excitation peak near 488 nm, making it suitable for standard blue lasers. Question 27. The principle of immunofluorescence microscopy relies on: A) Radioactive labeling of antigens B) Enzyme‑mediated color development C) Fluorophore‑conjugated antibodies binding to target antigens D) Light scattering by antigen‑antibody complexes Answer: C Explanation: Fluorescent tags emit light upon excitation, allowing visualization of antigen distribution. Question 28. Hybridoma technology is used to produce: A) Polyclonal antibodies from serum B) Recombinant cytokines C) Monoclonal antibodies of a single specificity D) Antigen‑presenting dendritic cells Answer: C Explanation: Fusion of a specific B cell with a myeloma cell creates a hybridoma that secretes identical antibodies. Question 29. Which vaccine type contains only the purified capsular polysaccharide of the pathogen? A) Live‑attenuated vaccine
B) Inactivated (killed) vaccine C) Subunit polysaccharide vaccine D) Recombinant protein vaccine Answer: C Explanation: Polysaccharide vaccines consist solely of the bacterial capsule, e.g., pneumococcal polysaccharide vaccine. Question 30. The primary advantage of mRNA vaccines over traditional protein subunit vaccines is: A) They require adjuvants for immunogenicity. B) They integrate into the host genome. C) They can be rapidly designed and produced. D) They induce only humoral immunity. Answer: C Explanation: Synthetic mRNA can be quickly generated from sequence data, enabling fast response to emerging pathogens. Question 31. Herd immunity threshold depends principally on: A) The pathogen’s incubation period B) The basic reproduction number (R₀) C) The age distribution of the population D) The seasonality of infection Answer: B Explanation: The proportion of immune individuals needed to interrupt transmission is 1 – 1/R₀. Question 32. Which type of hypersensitivity is mediated by immune complex deposition and complement activation?
Question 35. Primary immunodeficiency caused by a defect in the IL‑2Rγ chain leads to severe combined immunodeficiency (SCID) characterized by: A) Lack of B cells only B) Lack of T cells only C) Absence of both T and NK cells D) Overactive B cells producing autoantibodies Answer: C Explanation: γc chain mutations affect signaling for multiple cytokine receptors, impairing T‑cell and NK‑cell development. Question 36. Which laboratory test is most appropriate for diagnosing a recent infection with Helicobacter pylori by detecting IgM antibodies? A) Urea breath test B. Stool antigen ELISA C. Serum IgM ELISA D. Rapid urease test Answer: C Explanation: IgM ELISA can indicate recent exposure, whereas other tests detect active infection rather than serologic response. Question 37. The “window period” in HIV testing refers to: A) The time after infection when viral load is undetectable by PCR. B) The interval between exposure and appearance of detectable antibodies. C) The period when antiretroviral therapy is most effective. D) The time when HIV‑specific T cells are absent. Answer: B
Explanation: During the window period, antibodies have not yet risen to detectable levels, so nucleic‑acid tests are preferred. Question 38. Which immunoglobulin is most efficiently transferred across the placenta to provide passive immunity to the fetus? A) IgM B) IgG C) IgA D) IgE Answer: B Explanation: FcRn receptors mediate selective transport of IgG across the placental syncytiotrophoblast. Question 39. In the context of transplantation, graft‑versus‑host disease (GVHD) is primarily mediated by: A) Host B cells producing antibodies against donor antigens B. Donor T cells attacking host tissues C. Host NK cells lysing donor cells D. Complement activation by donor serum Answer: B Explanation: Donor‑derived T lymphocytes recognize host antigens as foreign and initiate inflammatory damage. Question 40. The “danger model” of immune activation proposes that: A) Self‑antigens are always ignored. B. Immune responses are triggered by recognition of pathogen‑associated molecular patterns (PAMPs) only.
A. Increase in the number of antibody molecules produced. B. Somatic hypermutation of variable region genes followed by selection of higher‑affinity clones. C. Switch from IgM to IgG isotype. D. Binding of antibodies to multiple epitopes simultaneously. Answer: B Explanation: Somatic hypermutation introduces point mutations; B cells with higher affinity receptors are preferentially expanded. Question 44. In a type IV hypersensitivity reaction, the key effector cells are: A. IgE‑bound mast cells B. Neutrophils C. CD4⁺ Th1 cells and macrophages D. Complement proteins Answer: C Explanation: Delayed‑type hypersensitivity is mediated by Th1 cytokines that activate macrophages. Question 45. Which laboratory technique is most suitable for detecting a single‑protein antigen in a complex mixture with high specificity? A. ELISA B. Western blot C. Agglutination test D. Complement fixation Answer: B Explanation: Western blot separates proteins by electrophoresis and uses specific antibodies to identify the target band.
Question 46. The primary immunologic purpose of the spleen’s marginal zone B cells is to: A. Produce high‑affinity IgG antibodies. B. Respond rapidly to blood‑borne polysaccharide antigens. C. Present antigen to CD8⁺ T cells. D. Generate regulatory T cells. Answer: B Explanation: Marginal zone B cells are specialized for T‑independent responses to encapsulated bacteria. Question 47. Which of the following cytokines is a potent inducer of class‑switch recombination to IgE? A. IL‑ 2 B. IL‑ 4 C. IL‑ 12 D. IFN‑α Answer: B Explanation: IL‑4 drives CSR toward IgE, facilitating allergic sensitization. Question 48. In the context of tumor immunology, the term “immune checkpoint” refers to: A. Physical barriers preventing T‑cell infiltration. B. Molecules that down‑regulate T‑cell activation (e.g., PD‑1, CTLA‑4). C. Antigen‑processing defects in tumor cells. D. Increased expression of MHC I on tumor cells. Answer: B
D. IgM is secreted in larger quantities than IgG. Answer: A Explanation: The planar arrangement of the five IgM monomers presents a single binding site for C1q, efficiently initiating the classical pathway. Question 52. A patient with a deficiency in the complement component C5b‑9 (membrane attack complex) would be most susceptible to infections with: A. Gram‑positive cocci B. Encapsulated bacteria C. Neisseria species D. Mycobacteria Answer: C Explanation: The MAC is critical for killing Neisseria; deficiencies predispose to meningococcal disease. Question 53. Which of the following molecules is a pattern‑recognition receptor (PRR) that detects bacterial lipopolysaccharide (LPS)? A. Toll‑like receptor 4 (TLR4) B. CD C. MHC II D. FcγRIII Answer: A Explanation: TLR4 specifically binds LPS, triggering innate immune signaling pathways. Question 54. The process by which dendritic cells migrate from peripheral tissue to lymph nodes is driven by: A. IL‑ 2
B. CCR7 chemokine receptor interaction with CCL19/CCL C. Complement C5a gradient D. Integrin α4β7 binding to VCAM‑ 1 Answer: B Explanation: CCR7 directs dendritic cells toward lymphoid chemokines CCL19 and CCL21. Question 55. In the context of vaccine adjuvants, alum primarily acts by: A. Activating TLR B. Inducing a strong Th1 response C. Forming a depot that slowly releases antigen and promotes inflammasome activation D. Serving as a viral vector Answer: C Explanation: Aluminum salts create an antigen depot and stimulate the NLRP3 inflammasome, enhancing antibody responses. Question 56. Which of the following best describes the “IgG subclass switching” that occurs during a secondary immune response? A. Switch from IgG1 to IgG2 only B. Switch from IgM to any IgG subclass depending on cytokine milieu C. Switch from IgA to IgE D. No subclass switching occurs after the primary response Answer: B Explanation: Cytokines such as IFN‑γ and IL‑4 influence which IgG subclass is produced during class switching. Question 57. The “danger signal” released by necrotic cells that activates dendritic cells is:
