Question 1

Functions of natural killer (NK) cells NK cells in innate immunity are best described as participating in which of the following processes?

Accepted Answer

Introduction / Context: Natural killer (NK) cells are cytotoxic lymphocytes of the innate immune system. They provide rapid responses against virally infected cells and surveillance against transformed (tumour) cells. Understanding their roles clarifies early host defense and interactions with adaptive immunity during transplantation and infection. Given Data / Assumptions: NK cells kill targets lacking or downregulating MHC class I molecules (the “missing self” concept). They mediate antibody-dependent cellular cytotoxicity via FcγRIII (CD16). Cytotoxicity uses perforin/granzyme pathways and death receptors. Concept / Approach: In tumour rejection, many cancers reduce MHC I to evade CD8 T cells, making them NK targets. In viral infection, cells often alter MHC I or express stress ligands recognized by NK activating receptors, driving non-specific killing of virus-transformed cells. In transplantation, innate mechanisms including NK cells can contribute to allograft injury, especially when MHC disparities reduce inhibitory signaling; NK cells also modulate adaptive responses via cytokines (IFN-γ, TNF) and crosstalk with dendritic cells. Step-by-Step Solution: Identify NK triggers: missing self and induced self (stress ligands).Map triggers to outcomes: tumour surveillance and antiviral cytotoxicity.Acknowledge NK participation in allograft responses under MHC mismatch conditions. Verification / Alternative check: Experimental models show NK-mediated lysis of MHC I–deficient targets; clinical observations link NK activity with viral control and with roles in graft outcomes, supporting their involvement across scenarios listed. Why Other Options Are Wrong: Choosing only one role underestimates NK versatility. “None of these” conflicts with well-established NK functions. Common Pitfalls: Assuming allograft rejection is exclusively T cell–mediated; innate components, including NK cells, contribute. Confusing NK cells with NKT cells or cytotoxic T lymphocytes. Final Answer: All of these

Question 2

Definition of isoantigens (alloantigens) Which statement correctly describes isoantigens (also called alloantigens) within a species?

Accepted Answer

Introduction / Context: Immunology distinguishes antigens by distribution across and within species. Correctly defining isoantigens (alloantigens) is fundamental for understanding blood groups, transplantation, and population genetics. These antigens vary among individuals of the same species and elicit immune responses in nonidentical members. Given Data / Assumptions: Species-level vs. individual-level antigen distribution. Clinical relevance in transfusion and graft compatibility. Terminology: isoantigen = alloantigen; xenoantigen = heterophilic between species; autoantigen = self-antigen. Concept / Approach: Isoantigens are polymorphic antigens present in some, but not all, individuals of the same species. Classic examples include ABO and Rh blood group antigens and histocompatibility antigens (HLA) in humans. Because expression varies, introducing cells or tissues carrying a nonshared isoantigen can provoke an immune response in a recipient lacking that antigen. Step-by-Step Solution: Identify scope: within a single species.Determine prevalence: present in some individuals, absent in others.Select the option reflecting this polymorphic distribution. Verification / Alternative check: Transfusion practice requires matching ABO/Rh isoantigens; transplantation compatibility testing matches donor and recipient HLA, directly illustrating isoantigen concepts. Why Other Options Are Wrong: “All members of a species”: That describes species-common antigens, not isoantigens. “Different species” options: Refer to xenogenic differences, not alloantigens. “Only in plants”: Not relevant; isoantigens are defined across taxa, including humans. Common Pitfalls: Confusing alloantigens with autoantigens (self) or xenoantigens (between species). Final Answer: Antigens found in some members of a species

Question 3

How antibody diversity is generated Against a single antigen with many epitopes, how do vertebrates generate numerous antibody specificities?

Accepted Answer

Introduction / Context: Antibody diversity underpins adaptive immunity. A single antigen typically has multiple epitopes, and the immune system produces many B-cell clones, each with a unique receptor/antibody. Understanding the genetic mechanisms producing this diversity is essential in immunology and vaccine design. Given Data / Assumptions: Immunoglobulins have heavy (H) and light (L) chains with variable (V) and constant (C) regions. Somatic DNA rearrangement occurs in developing B cells. Affinity maturation can occur after antigen exposure via somatic hypermutation. Concept / Approach: Primary diversity arises from V(D)J recombination (rearrangements of V, D, and J gene segments for heavy chains; V and J for light chains) mediated by RAG1/2, plus junctional diversity (P/N nucleotide addition) and imprecise joining. Combinatorial diversity results from pairing different heavy and light chains. Secondary diversification involves somatic hypermutation and class-switch recombination (AID-mediated), further expanding specificity and affinity. Step-by-Step Solution: Identify mechanism (a): V(D)J rearrangement and junctional changes create diverse V regions.Identify mechanism (b): Random pairing of many heavy with many light chains multiplies possibilities.Exclude (c): antibodies do not promiscuously “reshape” to any epitope; binding specificity is encoded by sequence and structure. Verification / Alternative check: Sequencing of B-cell receptors shows unique V(D)J recombination patterns; knockout of RAG or AID abrogates diversity, confirming mechanisms (a) and (b) are essential. Why Other Options Are Wrong: (c) suggests induced-fit to any epitope; while minor conformational changes occur, they do not create arbitrary new specificities. “Neither”: contradicts established genetics of immunoglobulins. Common Pitfalls: Confusing class switching (changes constant region) with specificity generation (variable region). Final Answer: Both (a) and (b)

Question 4

Protein targeting signals Which protein would typically NOT require a localization (targeting) signal peptide or sequence to reach its final destination in the cell?

Accepted Answer

Introduction / Context: Proteins are synthesized on ribosomes and must be targeted to correct cellular compartments. Signal sequences or localization motifs direct proteins to the nucleus, ER, mitochondria, peroxisomes, and other destinations. Understanding which proteins do and do not require such signals is crucial for cell biology and biotechnology. Given Data / Assumptions: Cytosolic proteins function in the cytosol where they are synthesized. Targeting can be co-translational (to ER via signal peptide/SRP) or post-translational (e.g., NLS to nucleus). Signals vary: NLS, signal peptide, transit peptides, peroxisomal PTS1/PTS2, and sorting tags like M6P for lysosomes. Concept / Approach: Proteins remaining in the cytosol generally lack dedicated signal sequences; default localization is the cytosol. In contrast, nuclear proteins require nuclear localization signals (NLS). Secreted and membrane proteins need an N-terminal signal peptide to enter the ER, followed by trafficking. Lysosomal enzymes acquire mannose-6-phosphate tags in the Golgi for targeting to lysosomes. ER membrane proteins possess topogenic sequences (signal-anchor, stop-transfer) to insert into membranes. Step-by-Step Solution: List destinations requiring signals: nucleus (NLS), ER/secretory pathway (signal peptide), mitochondria/peroxisomes (transit/PTS), lysosome (M6P).Identify the default: cytosolic residence with no targeting signal.Choose “cytosolic protein” as the one not needing a localization signal. Verification / Alternative check: Expression of signal-less GFP remains cytosolic; adding NLS drives nuclear import; adding signal peptide targets to ER—experimental evidence for signal dependency. Why Other Options Are Wrong: Nuclear proteins: require NLS for import via importins. ER-bound and cell-surface proteins: require ER signal sequences and membrane topogenic signals. Lysosomal hydrolases: require M6P tagging for proper sorting. Common Pitfalls: Assuming diffusion suffices for nuclear entry; macromolecules need active transport and NLS. Final Answer: A cytosolic protein

Question 5

Determinants of antigenicity Which factors primarily determine whether a substance will be antigenic (capable of eliciting an adaptive immune response)?

Accepted Answer

Introduction / Context: Antigenicity depends on several properties of a molecule. Appreciating these determinants helps explain why some substances are highly immunogenic while others are poor antigens unless coupled to carriers (haptens). This knowledge informs vaccine design and diagnostic assay development. Given Data / Assumptions: Molecules vary in chemical class (proteins, polysaccharides, lipids, nucleic acids). Physical factors like size and complexity influence immunogenicity. Host recognition of “non-self” is central to adaptive responses. Concept / Approach: Strong antigens are typically high-molecular-weight, complex proteins or polysaccharides with diverse epitopes. Small molecules (haptens) are not immunogenic unless conjugated to carriers. Foreignness (phylogenetic distance) increases likelihood of recognition; self molecules are usually tolerated. Chemical nature matters because proteins can be processed and presented effectively by MHC, eliciting T-dependent responses; polysaccharides often elicit T-independent responses with limited memory/class switching. Step-by-Step Solution: Assess chemical nature: proteins > polysaccharides > lipids/nucleic acids (alone) as antigens.Assess size/complexity: larger, more complex structures present multiple epitopes.Assess foreignness: greater difference from host increases antigenicity. Verification / Alternative check: Empirical vaccine performance (e.g., protein toxoids) and hapten-carrier experiments validate the roles of chemical nature, size, and foreignness in dictating immunogenicity. Why Other Options Are Wrong: Choosing any single factor ignores multifactorial reality. “None” contradicts abundant immunological evidence. Common Pitfalls: Equating antigenicity with immunogenicity in all contexts; doses, route, and adjuvants also matter. Final Answer: All of these

Question 6

Genetic location of MHC Major Histocompatibility Complex (MHC) genes are clustered on which chromosomes in humans and in mice, respectively?

Accepted Answer

Introduction / Context: The Major Histocompatibility Complex encodes molecules that present antigenic peptides to T cells and shape immune responses. Knowing the chromosomal location of the MHC is fundamental for genetics, transplantation biology, and disease association studies (HLA typing in humans). Given Data / Assumptions: Human MHC is called HLA (Human Leukocyte Antigen). Mouse MHC is called H-2. We are asked for chromosomal locations in each species. Concept / Approach: The HLA complex resides on human chromosome 6 (short arm, 6p21). The murine H-2 complex is on mouse chromosome 17. These loci are highly polymorphic and include class I (A, B, C in humans), class II (DP, DQ, DR), and class III regions. Their gene products are central to antigen presentation and graft compatibility. Step-by-Step Solution: Recall: HLA → human chr 6.Recall: H-2 → mouse chr 17.Match with the option listing 6 (human) and 17 (mouse). Verification / Alternative check: Standard immunology references and genetic maps place HLA on 6p21 and H-2 on mouse 17, widely used in synteny and QTL studies. Why Other Options Are Wrong: Other chromosome pairings invert or misassign species. Chromosome 1 for both species is incorrect for MHC. Common Pitfalls: Confusing numerical chromosome labels between species; remember “6 for HLA, 17 for H-2.” Final Answer: Chromosome 6 in humans, chromosome 17 in mice

Question 7

Antigen presentation by macrophages Human macrophages are professional antigen-presenting cells. Through which molecules do they present processed peptide antigens to T cells?

Accepted Answer

Introduction / Context: Antigen-presenting cells (APCs) such as macrophages, dendritic cells, and B cells process antigens and present peptide fragments to T cells. The molecular platform for presentation in humans is HLA (the human MHC). Understanding these interactions underlies vaccine design and immunotherapy. Given Data / Assumptions: Macrophages present peptides derived from phagocytosed or endocytosed antigens. CD4 T cells recognize peptides bound to class II HLA; CD8 recognize peptides bound to class I HLA. Accessory molecules and cytokines support activation but are not the presenting platform. Concept / Approach: HLA class II molecules (HLA-DP, -DQ, -DR) present exogenous peptides to CD4 T helper cells after processing in endosomal compartments. HLA class I (HLA-A, -B, -C) presents endogenous peptides to CD8 cytotoxic T cells. Macrophages express both classes (especially after activation) and prime/activate T cells in coordination with costimulatory signals (e.g., CD80/CD86) and cytokines. Step-by-Step Solution: Identify the platform of peptide presentation: HLA molecules.Exclude structural proteins (actin) and signaling cytokines (interleukins) as presenters.Recognize CD8 as a T-cell coreceptor, not the presenting molecule. Verification / Alternative check: Flow cytometry shows upregulation of HLA-DR on activated macrophages; functional assays demonstrate T-cell activation dependent on peptide–HLA complexes. Why Other Options Are Wrong: Actin: cytoskeletal protein; not involved in antigen presentation on the cell surface. Interleukin: soluble mediators; they modulate responses but do not present peptides. CD8: coreceptor on T cells; recognizes HLA class I, not a presenter. TLRs: pattern-recognition receptors; initiate innate signaling, not peptide display. Common Pitfalls: Confusing costimulation and cytokines with the peptide–MHC presentation mechanism. Final Answer: HLA (MHC molecules)

Question 8

Polymorphonuclear leukocytes (PMNs) Which statements accurately describe PMNs (e.g., neutrophils) in host defense?

Accepted Answer

Introduction / Context: Polymorphonuclear leukocytes (PMNs), primarily neutrophils in humans, are first responders in acute inflammation. Recognizing their origin, recruitment, and effector functions is fundamental to understanding innate immunity and clinical indicators like neutrophilia in infection. Given Data / Assumptions: Complement activation generates chemoattractants (e.g., C5a). Hematopoiesis in bone marrow produces myeloid lineages, including neutrophils. Neutrophils perform phagocytosis and microbial killing. Concept / Approach: Complement fragments (C3a, C5a) and chemokines (e.g., IL-8/CXCL8) guide PMNs to infection sites. Neutrophils originate from hematopoietic stem cells via granulopoiesis and circulate briefly before migrating into tissues. Their antimicrobial arsenal includes reactive oxygen species (respiratory burst via NADPH oxidase), lysosomal enzymes, antimicrobial peptides, and formation of neutrophil extracellular traps (NETs). Defects (e.g., chronic granulomatous disease) illustrate their importance. Step-by-Step Solution: Confirm bone-marrow derivation of PMNs (myeloid lineage).Identify chemotaxis by complement-derived C5a and chemokines.Affirm phagocytosis and intracellular killing as hallmark functions. Verification / Alternative check: Clinical labs show neutrophil recruitment to inflamed tissues; genetic or functional deficits in PMNs result in recurrent infections, validating their central role. Why Other Options Are Wrong: “None of the above” contradicts well-established PMN biology. Choosing only one statement underestimates their multifaceted role. Common Pitfalls: Confusing PMNs with mononuclear phagocytes (macrophages); both are phagocytic but differ in lifespan and roles. Final Answer: All of the above

Question 9

In clinical immunology, which cell type characteristically expresses the CD3 surface antigen on its membrane (a pan-T-cell marker used in flow cytometry and diagnostics)?

Accepted Answer

Introduction / Context: CD markers (cluster of differentiation) are cell-surface molecules used to identify and classify immune cells. CD3 is a classic pan-T-cell marker tightly associated with the T-cell receptor (TCR) complex. This question assesses recognition of core immunophenotyping markers used in diagnostics and research. Given Data / Assumptions: We are comparing common leukocyte subsets: T cells, B cells, granulocytes, monocytes, and NK cells. We focus specifically on the presence of CD3 at the cell surface. Standard human immunology conventions are used (flow cytometry panels). Concept / Approach: CD3 is a multi-chain signaling complex (gamma, delta, epsilon, zeta) noncovalently associated with the TCR. It is essential for signal transduction after antigen recognition. Therefore, all mature T cells (both CD4+ and CD8+) express CD3. Other leukocytes do not display CD3 under normal conditions. Step-by-Step Solution: Step 1: Recall that CD3 is part of the TCR complex → present on all T-lymphocytes.Step 2: Evaluate other leukocytes. B cells express CD19/CD20/CD21; NK cells are CD16+ and CD56+ (CD3−); monocytes are CD14+; granulocytes are CD15+/CD66b+.Step 3: Therefore, the only correct choice is T cells. Verification / Alternative check: Standard clinical flow cytometry panels gate lymphocytes by scatter, then CD3 versus CD19/CD20. T cells are CD3+; B cells are CD3− and CD19+/CD20+. NK cells appear CD3− CD56+/CD16+. Why Other Options Are Wrong: Granulocytes: No TCR/CD3 complex; typically CD15+/CD66b+. Monocytes: CD14 high; lack CD3. B cells: CD19/CD20, not CD3. NK cells: CD16/CD56; specifically CD3 negative. Common Pitfalls: Confusing CD3 (pan-T) with CD4/CD8 (subsets). Assuming NK-T cells are NK cells; invariant NKT are CD3+ (still T-lineage), whereas classic NK cells are CD3−. Final Answer: T cells (all mature T-lymphocytes)

Question 10

CD8 surface antigen is a co-receptor characteristic of which T-cell subsets in human immunology?

Accepted Answer

Introduction / Context: CD8 is a co-receptor that binds MHC class I and is central to cytotoxic T-cell function. Historically, “suppressor T cells” were also linked to CD8 expression in older literature, while modern terminology distinguishes CD8+ cytotoxic T lymphocytes and CD4+ regulatory T cells. The question reflects the legacy phrasing often seen on exams. Given Data / Assumptions: We are using classical exam conventions where suppressor T cells are grouped with CD8 positivity. CD8 pairs with the TCR to recognize peptide–MHC class I. Concept / Approach: CD8 engagement stabilizes TCR–peptide–MHC I interactions and recruits signaling molecules. Most CD8+ cells are cytotoxic effectors or memory cells. Historical “Ts” (suppressor) were usually CD8+, although modern regulatory T cells (Treg) are primarily CD4+ CD25+ FoxP3+. Step-by-Step Solution: Step 1: Identify the definitive CD8+ lineage → cytotoxic T cells.Step 2: Map historical terminology: suppressor T cells were classically CD8+.Step 3: Select the inclusive option that matches exam convention: both (a) and (b). Verification / Alternative check: Flow cytometry profiles show CD3+CD8+ for Tc cells. Older immunology texts list CD8 as a marker for cytotoxic/suppressor populations; contemporary Treg are CD4+ FoxP3+. Why Other Options Are Wrong: Only cytotoxic: ignores the legacy suppressor designation used in some question banks. Only suppressor: incomplete. None or B/NK: incorrect; CD8 is T-lineage specific (with rare exceptions such as some NK subsets expressing CD8 alpha, but not as classical T-cell CD8αβ co-receptor). Common Pitfalls: Mixing modern Treg (CD4+FoxP3+) with old “suppressor” terminology. Final Answer: Both cytotoxic (Tc) and suppressor subsets (historical term)

Question 11

For which infectious disease is passive immunization routinely used in clinical practice to provide immediate neutralization (e.g., administration of antitoxin or immunoglobulin)?

Accepted Answer

Introduction / Context: Passive immunization involves giving pre-formed antibodies (antitoxin or immunoglobulin) to rapidly neutralize a toxin or pathogen. It provides immediate, short-term protection, unlike vaccines that induce active immunity. Diphtheria is a classic indication for passive antitoxin therapy in suspected cases. Given Data / Assumptions: We seek a disease where immediate antibody-mediated neutralization is standard. Passive immunity does not require the host to mount a response. Concept / Approach: Corynebacterium diphtheriae produces a potent exotoxin causing local necrosis and systemic complications. Diphtheria antitoxin (DAT) neutralizes circulating toxin and is most effective when given early. Tuberculosis and typhoid rely on antimicrobial therapy and vaccines; passive immunization has no routine role. Step-by-Step Solution: Step 1: Identify toxin-mediated illnesses where antitoxin is lifesaving → diphtheria, tetanus, botulism.Step 2: Compare with TB and typhoid → primarily treated with antibiotics and prevented by vaccines; passive antibodies are not standard of care.Step 3: Choose diphtheria as the correct example. Verification / Alternative check: Clinical guidelines recommend immediate DAT administration in suspected diphtheria, alongside antibiotics, to reduce morbidity and mortality. Why Other Options Are Wrong: Tuberculosis: No role for passive antibodies; management is multi-drug therapy and BCG vaccination (active). Enteric fever: Treated with antibiotics; vaccines used for prevention. Measles: Post-exposure immune globulin may be used in certain high-risk cases, but the canonical textbook answer in this set is diphtheria (antitoxin). All of these: Incorrect because TB/typhoid lack routine passive therapy. Common Pitfalls: Confusing post-exposure prophylaxis (e.g., rabies, hepatitis B) with diseases not treated by passive immunization. Final Answer: Diphtheria

Question 12

In mucosal immunity of the small intestine, what is the primary role of specialized microfold (M) cells located over Peyer’s patches?

Accepted Answer

Introduction / Context: M cells are specialized epithelial cells that overlay Peyer’s patches in the gut-associated lymphoid tissue (GALT). They are crucial for sampling luminal antigens and initiating mucosal immune responses. Understanding their function helps explain oral tolerance and mucosal vaccine strategies. Given Data / Assumptions: Location: Follicle-associated epithelium of the ileum. Function: Antigen sampling and delivery to immune cells. Concept / Approach: M cells lack a thick brush border and mucus production; instead, they efficiently endocytose or phagocytose luminal particles and deliver them via transcytosis to the immune cell–rich basolateral pocket containing macrophages and dendritic cells. This promotes antigen presentation and activation of B and T cells. Step-by-Step Solution: Step 1: Identify that M cells are not ciliated and do not secrete large amounts of mucus.Step 2: Recall their unique function: transport of intact antigens to APCs.Step 3: Select the option describing transcytosis to underlying macrophages/DCs. Verification / Alternative check: Histology shows M-cell basolateral pockets with immune cells. Oral vaccines (e.g., live attenuated) exploit this pathway for uptake. Why Other Options Are Wrong: Mucus trapping and ciliary propulsion: Functions of goblet and ciliated respiratory epithelium, not intestinal M cells. Apoptosis induction: Not the primary role of M cells. IgA secretion: Plasma cells in lamina propria secrete IgA, which is transported across enterocytes via pIgR. Common Pitfalls: Confusing M cells with enterocytes or goblet cells. Assuming all mucosal epithelial cells have similar roles. Final Answer: Transcytose microbes to underlying macrophages/dendritic cells for processing and antigen presentation

Question 13

Protective immunity is typically not long-lasting against which of the following infections, largely due to frequent antigenic variation?

Accepted Answer

Introduction / Context: Duration of immunity varies by pathogen. Influenza viruses undergo antigenic drift (and occasionally shift), leading to changes in surface antigens hemagglutinin and neuraminidase. As a result, prior immunity may provide limited protection, necessitating periodic vaccine updates. Given Data / Assumptions: We compare typical durability of immunity after infection or vaccination. Focus on pathogens known for antigenic change. Concept / Approach: Antigenic drift results from point mutations altering epitopes; antigenic shift (reassortment) can produce novel subtypes. Consequently, long-term sterilizing immunity is uncommon, and seasonal vaccines are reformulated annually to match circulating strains. Step-by-Step Solution: Step 1: Identify which diseases exhibit rapid antigenic evolution → influenza.Step 2: Contrast with mumps, diphtheria, and pertussis, which generally produce longer-lasting immunity (though pertussis immunity can wane, it is not due to rapid antigenic drift comparable to influenza).Step 3: Select influenza. Verification / Alternative check: Public health recommendations call for annual influenza vaccination, confirming limited durability of strain-specific immunity. Why Other Options Are Wrong: Pertussis: Immunity wanes over years, but the classic exam answer for “not long-lasting due to frequent antigenic variation” is influenza. Diphtheria and mumps: Vaccination confers durable protection in most individuals with boosters as scheduled; antigenic drift is not a dominant issue. Rubella: Generally durable vaccine-induced immunity. Common Pitfalls: Equating waning immunity from lack of boosters with true antigenic variation. Final Answer: Influenza

Question 14

Gastrointestinal host defenses: which option is NOT a typical defense mechanism of the colon?

Accepted Answer

Introduction / Context: The colon maintains a robust defensive environment to contain a dense microbial community while preventing pathogen overgrowth. Understanding which mechanisms are present (and which are not) is fundamental to GI mucosal immunology and infection control. Given Data / Assumptions: Anatomy: Large intestine with slow transit compared to small intestine. Defenses: Mucus, sIgA, microbiota, barrier integrity. Concept / Approach: Key defenses in the colon include a thick mucus layer, abundant secretory IgA, antimicrobial peptides, epithelial tight junctions, and, importantly, colonization resistance from the resident microbiota. Unlike the small intestine, the colon generally exhibits slower flow and mixing, not a “fast flow rate.” Step-by-Step Solution: Step 1: List true defenses: sIgA, mucus, epithelial barrier, and microbiota.Step 2: Identify the distractor: “fast flow rate,” which is not characteristic of the colon and would not be considered a defense there.Step 3: Choose the non-defense option: fast luminal flow rate. Verification / Alternative check: Physiology texts describe slower transit in the colon to allow water absorption and fermentation; defensive strategies rely on mucus and immunologic factors rather than rapid clearance. Why Other Options Are Wrong: sIgA: Neutralizes pathogens and toxins without inflammation. Microbiota: Competes with pathogens (colonization resistance). Mucus: Physical barrier and matrix for antimicrobial molecules. Tight junctions: Maintain barrier integrity. Common Pitfalls: Assuming all parts of the gut use rapid flow as a defense; this applies more to the small intestine. Final Answer: Fast luminal flow rate

Question 15

Select the correct statement about adaptive immune recognition and gene generation mechanisms:

Accepted Answer

Introduction / Context: Adaptive immunity relies on vast receptor diversity generated somatically. T-cell receptors (and B-cell receptors/antibodies) arise through V(D)J recombination, junctional diversity, and, for B cells, somatic hypermutation after activation. By contrast, MHC molecules are encoded by polymorphic germline genes without somatic rearrangement. Given Data / Assumptions: TCR and BCR diversity mechanisms are compared to MHC genetics. Basic definitions of antigen, epitope, and lineage markers are assumed. Concept / Approach: TCR α/β chains are formed by recombining variable (V), diversity (D, beta chain only), and joining (J) segments with random N/P nucleotide additions. This produces a unique receptor per T cell. MHC genes are inherited and highly polymorphic but do not undergo somatic rearrangement. Not all T cells are CD4+; many are CD8+. Step-by-Step Solution: Step 1: Identify which statement reflects somatic recombination → TCR generation.Step 2: Exclude incorrect claims: “all T cells CD4+” is false; “epitope has multiple antigens” reverses the relationship (an antigen has multiple epitopes); “MHC II by rearrangement” is false; “BCR fixed germline” is false.Step 3: Choose statement (c). Verification / Alternative check: Immunology texts detail RAG1/2-mediated recombination for TCR/BCR; MHC genes are non-rearranging, though polymorphic at the population level. Why Other Options Are Wrong: CD4+ only: CD8+ cytotoxic T cells exist. Epitope vs antigen: An antigen contains many epitopes; not vice versa. MHC II rearrangement: Does not occur somatically. BCR fixed: BCRs undergo V(D)J recombination and, after activation, somatic hypermutation. Common Pitfalls: Confusing population polymorphism (MHC) with somatic recombination (TCR/BCR). Final Answer: T-cell receptors (TCRs) are generated by random assortment and recombination of gene segments (V(D)J recombination)

Question 16

Exposure of human lymphocytes to bacterial endotoxin (lipopolysaccharide, LPS) typically causes blast transformation in which population?

Accepted Answer

Introduction / Context: Mitogens are substances that non-specifically stimulate lymphocyte proliferation. Bacterial endotoxin (LPS) is a classical B-cell mitogen in many species, driving polyclonal B-cell activation and blast formation. Understanding which stimuli activate which lymphocyte subsets is key in immunology labs and diagnostics. Given Data / Assumptions: Mitogen: LPS from Gram-negative bacteria. Cellular targets: B cells vs T cells vs monocytes. Concept / Approach: LPS engages pattern-recognition receptors (e.g., TLR4-MD2-CD14 complex) and can directly activate B cells (especially in mice) leading to blast transformation and proliferation. T-cell mitogens are different (e.g., phytohemagglutinin, concanavalin A, anti-CD3 antibodies). Monocytes respond to LPS with cytokine secretion rather than classic blast proliferation. Step-by-Step Solution: Step 1: Identify LPS as a B-cell mitogen.Step 2: Recall T-cell mitogens are lectins or anti-CD3; LPS is not primarily T-cell mitogenic.Step 3: Choose B cells as the population undergoing blast transformation with LPS. Verification / Alternative check: In vitro assays show robust B-cell proliferation with LPS; monocytes produce TNF and IL-1 in response but do not show classic lymphoblast proliferation. Why Other Options Are Wrong: T cells: Not directly driven to blasts by LPS. Monocytes: Activate and secrete cytokines; not classic mitogen-induced blasts. None/Eosinophils: Incorrect; B-cell activation is well documented. Common Pitfalls: Assuming any strong innate stimulus is a T-cell mitogen; specificity matters. Final Answer: B cells (polyclonal activation)

Question 17

CD4+ T-cell receptors recognize peptides in the context of which antigen-presenting molecule class?

Accepted Answer

Introduction / Context: T-cell antigen recognition requires peptides presented by major histocompatibility complex (MHC) molecules. CD4+ helper T cells are restricted to antigens displayed by MHC class II, typically found on professional antigen-presenting cells (APCs) like dendritic cells, macrophages, and B cells. Given Data / Assumptions: Cell subset: CD4+ T cells. Antigen form: peptide bound to an MHC molecule. Concept / Approach: MHC class II presents exogenous peptides processed in endosomal/lysosomal compartments. CD4 acts as a co-receptor by binding to non-polymorphic regions of MHC II, stabilizing the TCR–peptide–MHC interaction and facilitating signaling via the CD3 complex. Step-by-Step Solution: Step 1: Recall restriction: CD4+ → MHC II; CD8+ → MHC I.Step 2: Eliminate options: “no MHC required” is false for αβ TCRs; MHC I is for CD8+; secreted receptors refers to antibodies, not TCR; CD1 specializes in lipids and NKT cells, not general CD4+ peptide recognition.Step 3: Select MHC class II. Verification / Alternative check: APC-mediated antigen presentation to CD4+ T cells in lymph nodes is the cornerstone of helper T-cell priming. Why Other Options Are Wrong: MHC I: Restricts CD8+ cytotoxic T cells. No MHC/Secreted receptors: Describe B-cell antibodies, not TCR recognition. CD1: Presents lipids to invariant NKT, not conventional CD4+ peptide recognition. Common Pitfalls: Confusing the roles of CD4 and CD8 co-receptors. Final Answer: MHC class II molecules

Question 18

Which statement correctly describes CD8 T cells and their effector function in antiviral immunity?

Accepted Answer

Introduction / Context: CD8+ cytotoxic T lymphocytes (CTLs) are key effectors against intracellular pathogens, especially viruses. They survey MHC class I–presented peptides and kill infected targets through focused, contact-dependent mechanisms that limit bystander damage. Given Data / Assumptions: Effector mechanisms include perforin/granzyme exocytosis and Fas–FasL engagement. Antigen recognition is peptide–MHC I–restricted. Concept / Approach: Upon TCR recognition of foreign peptide on MHC I, CTLs form an immunological synapse with the target cell and deliver lethal hits through granule exocytosis or death receptor signaling. They do not secrete antibodies; that is a B-cell function. Both CD4 and CD8 thymocytes undergo positive and negative selection during development. Step-by-Step Solution: Step 1: Identify the hallmark function: direct, contact-dependent cytotoxicity of infected cells.Step 2: Exclude distractors: B-cell–like recognition of native antigens is incorrect for T cells; thymic selection statements are false simplifications; “only virus-infected” is too restrictive (they can also target tumor cells or cells with intracellular bacteria).Step 3: Choose the accurate description of CTL effector action. Verification / Alternative check: Immunology references detail CTL killing kinetics, immunological synapse, and serial killing capacity of CD8+ T cells. Why Other Options Are Wrong: Only virus-infected: Too narrow; any MHC I–presented foreign peptide (including tumor neoantigens) can be targeted. Same epitopes as B cells: B cells recognize native conformations; T cells recognize processed peptides on MHC. Thymic selection claim: Both lineages undergo positive and negative selection. Antibody secretion: A B-cell function, not CD8+ T-cell. Common Pitfalls: Confusing humoral and cellular effectors. Overgeneralizing targets of CD8+ T cells. Final Answer: CD8 T cells can kill individual virus-infected cells via contact-dependent cytotoxicity (perforin/granzyme or Fas–FasL pathways)

Question 19

Immunology – effector cells against helminths (large extracellular parasites) In host defense, which leukocyte type is primarily responsible for killing large parasites such as helminths via extracellular degranulation (e.g., major basic protein, eo…

Accepted Answer

Introduction / Context: Helminths are multicellular parasites that are too large to be phagocytosed. The immune system therefore relies on extracellular killing mechanisms. Understanding which leukocytes specialize in anti-helminth activity is high-yield in immunology and parasitology. Given Data / Assumptions: Target organisms are large extracellular worms (e.g., Ascaris, Schistosoma). Question asks for the cell type that kills by releasing toxic granule contents onto the parasite surface. Consider classical antibody-dependent mechanisms (IgE/IgG) and Th2 cytokines (IL-5). Concept / Approach: Eosinophils bind to IgE- or IgG-coated helminths via Fc receptors and release toxic proteins (major basic protein, eosinophil cationic protein, eosinophil peroxidase) and reactive oxygen species. IL-5 from Th2 cells drives eosinophil maturation and activation. This extracellular degranulation damages parasite integument and facilitates expulsion. Step-by-Step Solution: Identify helminths as too large for phagocytosis.Recall Th2/IL-5 bias and IgE-mediated recognition of parasites.Map the effector: eosinophils release major basic protein that is toxic to helminths.Select eosinophils as the primary extracellular killers in this context. Verification / Alternative check: Eosinophilia is a hallmark laboratory finding in tissue-invasive helminth infections; anti-IL-5 therapies reduce eosinophil counts and can diminish helminthic cytotoxic responses, supporting the mechanism. Why Other Options Are Wrong: Basophils: important for immediate hypersensitivity and IgE support but not principal helminth killers. Monocytes: differentiate to macrophages; mainly phagocytose smaller targets. Neutrophils: key for bacteria/fungi; limited role against large helminths. NK cells: kill virus-infected or tumor cells, not specialized for helminths. Common Pitfalls: Assuming “more granules” means better for all pathogens; granule content specificity matters. Confusing basophils with eosinophils in Type II/III vs. Type I hypersensitivity contexts. Final Answer: Eosinophils

Question 20

Antigen presentation – identifying non-APCs Which of the following is NOT considered a professional antigen-presenting cell (APC) capable of priming naïve T cells under physiologic conditions?

Accepted Answer

Introduction / Context: Professional antigen-presenting cells (APCs) process antigens and present peptide–MHC to T cells with necessary costimulation to initiate adaptive immunity. Knowing which cells are true professional APCs is crucial for understanding vaccine responses and immune tolerance. Given Data / Assumptions: Professional APCs express high levels of MHC and costimulatory molecules (e.g., CD80/CD86). Candidates listed include B cells, dendritic cells, PMNs, and macrophages. Task is to identify the non-APC among them. Concept / Approach: Dendritic cells, macrophages, and B cells are classic professional APCs. They can activate naïve T cells (especially dendritic cells) and present antigens effectively. Polymorphonuclear leukocytes (neutrophils/PMNs) are innate effector phagocytes; while they can internalize pathogens, they generally lack the sustained MHC II and costimulatory expression required for priming naïve T cells. Step-by-Step Solution: List professional APCs: dendritic cell, macrophage, B cell.Evaluate PMNs: potent phagocytes but poor antigen presenters for naïve T-cell priming.Select PMN as the non-APC. Verification / Alternative check: Experimental T-cell priming assays show robust activation by dendritic cells and, to lesser degrees, macrophages and B cells; PMNs do not reliably prime naïve T cells despite limited antigen presentation capacities in certain contexts. Why Other Options Are Wrong: B cell: can present antigen via BCR-mediated uptake to CD4 T cells. Dendritic cell: most potent professional APC. Macrophage: presents to effector/memory T cells and provides cytokines. “All of the above”: incorrect because several listed are APCs. Common Pitfalls: Equating phagocytosis with professional antigen presentation; costimulation is required for priming. Final Answer: Polymorphonuclear leukocyte (PMN)

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