Question 1

Innate defenses – lysozyme distribution Lysozyme, the peptidoglycan-cleaving enzyme of innate immunity, is characteristically abundant in which of the following human body fluids?

Accepted Answer

Introduction / Context: Lysozyme is an antimicrobial enzyme that cleaves the glycan backbone of bacterial peptidoglycan, compromising bacterial cell walls. Recognizing where lysozyme is abundant helps understand first-line barriers to infection at mucosal and glandular surfaces. Given Data / Assumptions: Question asks for a characteristic site with high lysozyme levels. Body fluids listed include saliva, sweat, cerebrospinal fluid (CSF), and urine. Single best answer format. Concept / Approach: Lysozyme is classically abundant in secretions that bathe exposed epithelia: tears, saliva, nasal secretions, and to a lesser extent sweat and milk. Saliva is a prototypical example used in textbooks to illustrate lysozyme-mediated protection in the oral cavity. CSF can contain lysozyme but it is not emphasized as a primary antimicrobial secretion, and urinary lysozyme levels vary and are typically low under normal physiology. Step-by-Step Solution: Identify the classic high-level secretions: tears and saliva.Among given options, saliva best matches the prototypical high lysozyme site.Select saliva as the most characteristic answer. Verification / Alternative check: Clinical saliva protein profiles consistently show lysozyme alongside lactoferrin and secretory IgA; antibacterial activity against Gram-positive organisms is well documented for saliva-derived lysozyme. Why Other Options Are Wrong: Sweat: contains lysozyme but at lower, variable levels; not as classic as saliva. CSF: may contain lysozyme, especially with inflammation, but is not a frontline external secretion. Urine: typically contains minimal lysozyme in healthy individuals; levels may rise pathologically. Bile: not a principal source of lysozyme. Common Pitfalls: Assuming “present anywhere” equals “characteristically abundant.” Exams usually expect tears or saliva as the canonical answers. Final Answer: Saliva

Question 2

Chemical nature of antigens From a chemical standpoint, an antigen capable of eliciting an adaptive immune response may be composed of which type(s) of biomolecule?

Accepted Answer

Introduction / Context: Antigens are substances recognized by the immune system, typically by antibodies or T-cell receptors after processing and presentation. Knowing the broad chemical classes that can be antigenic helps in vaccine design and diagnostic immunoassays. Given Data / Assumptions: Antigens vary widely in size and complexity. They may be T-dependent (often proteins) or T-independent (often polysaccharides). Lipids and nucleic acids can also be antigenic when appropriately presented or complexed. Concept / Approach: Proteins are generally the most immunogenic due to size, complexity, and ease of processing/presentation. Polysaccharides can be strong antigens (e.g., bacterial capsules) though often elicit T-independent responses. Lipids may be recognized via CD1-restricted T cells or when conjugated to proteins (haptens). Nucleic acids can be antigenic when complexed with proteins or in autoimmune settings. Therefore, multiple chemical classes can serve as antigens. Step-by-Step Solution: List major classes: proteins, polysaccharides, lipids, nucleic acids.Recall immunogenic hierarchy: proteins > polysaccharides > lipids/nucleic acids (alone).Select the inclusive option “Any of these.” Verification / Alternative check: Clinical vaccines include protein toxoids and polysaccharide (or conjugate) antigens; glycolipids are recognized by NKT cells, confirming multiple classes can be antigenic. Why Other Options Are Wrong: Restricting to any single class ignores diversity of antigens. “Nucleic acid only” is overly narrow and incorrect. Common Pitfalls: Confusing antigenicity (binding) with immunogenicity (ability to induce a response); context, size, and carriers matter. Final Answer: Any of these

Question 3

Fundamentals of B-cell biology Which of the following statements about B lymphocytes is FALSE?

Accepted Answer

Introduction / Context: B-cell development, tolerance, and effector functions are core immunology topics. Distinguishing where B cells mature and how they respond helps avoid common exam traps that confuse B-cell and T-cell pathways. Given Data / Assumptions: B cells and T cells arise from hematopoietic stem cells but mature in different primary lymphoid organs. Class switching changes isotype, not antigen specificity. Self-reactive immature B cells are negatively selected in bone marrow. Concept / Approach: B cells develop and undergo V(D)J recombination and central tolerance in the bone marrow. T cells develop in the thymus (T for thymus). After activation, B cells may class switch (e.g., IgM to IgG, IgA, or IgE) under T-helper and cytokine signals, and differentiate into plasma cells to secrete soluble antibodies. Thus, any statement sending B-cell development to the thymus is false. Step-by-Step Solution: Identify maturation sites: B = bone marrow; T = thymus.Evaluate each statement against standard B-cell biology.Select the thymus development claim as false. Verification / Alternative check: Clinical observations (e.g., agammaglobulinemia due to bone marrow B-cell defects) and thymic aplasia syndromes (e.g., DiGeorge) affecting T, not B, cells corroborate the distinct maturation sites. Why Other Options Are Wrong: Class switching (A) is correct and cytokine-driven. Surface immunoglobulin/BCR expression (C) is fundamental. Central tolerance/apoptosis (D) is accurate. Plasma cell differentiation (E) is a hallmark effector function. Common Pitfalls: Mixing up B-cell vs. T-cell maturation sites due to mnemonic confusion. Final Answer: Bone marrow stem cells migrate to the thymus and develop into B cells.

Question 4

T-cell markers – identifying cytotoxic T lymphocytes Cytotoxic T lymphocytes (killer T cells) are classically identified by which surface marker co-receptor?

Accepted Answer

Introduction / Context: CD markers distinguish immune cell subsets. Recognizing the marker for cytotoxic T lymphocytes (CTLs) is foundational for interpreting flow cytometry and understanding antigen recognition in cell-mediated immunity. Given Data / Assumptions: CD4 associates with MHC class II; CD8 with MHC class I. CTLs recognize peptides presented by MHC class I on most nucleated cells. Markers like CD25 indicate activation, not lineage. Concept / Approach: CTLs are CD8+ T cells. Their TCRs survey peptides bound to MHC class I and, upon activation, kill infected or transformed cells via perforin/granzyme and Fas–FasL pathways. CD4 identifies helper T cells (MHC II restricted) rather than CTLs. Step-by-Step Solution: Map CTL function (killing, MHC I restriction) to marker CD8.Exclude helper T marker CD4 and non-defining markers.Select CD8. Verification / Alternative check: Flow cytometry gating of CD3+CD8+ populations correlates with cytotoxic function; CD8 deficiency impairs viral clearance, validating the association. Why Other Options Are Wrong: CD4: helper T cells. CD7, CD9: not lineage-defining for CTLs. CD25: IL-2 receptor alpha; indicates activation/regulatory T cell features. Common Pitfalls: Equating activation markers with lineage markers. Final Answer: CD8

Immune System Questions