AP-Micro Immunology and Host Defense 2 — Questions and Answers
Question 1: Which cells present exogenous antigens via MHC class II molecules to activate CD4+ helper T cells?
- Cytotoxic T cells
- Plasma cells
- Professional antigen-presenting cells (APCs) (Correct answer)
- NK cells
Correct answer: Professional antigen-presenting cells (APCs)
Dendritic cells, macrophages, and B cells process exogenous antigens and present peptides on MHC class II to CD4+ T helper cells.
Question 2: Which immunoglobulin class is the most abundant in serum and the predominant antibody in secondary (anamnestic) immune responses?
- IgA
- IgM
- IgG (Correct answer)
- IgE
Correct answer: IgG
IgG constitutes ~75% of serum immunoglobulin and is the major antibody produced during secondary immune responses due to memory B cell activation.
Question 3: The first antibody class produced during a primary immune response, existing as a pentamer in serum, is:
- IgG
- IgA
- IgM (Correct answer)
- IgD
Correct answer: IgM
IgM is the earliest antibody produced in a primary response; its pentameric structure makes it highly effective at agglutination and complement activation.
Question 4: Which T cell subset directly kills virus-infected cells by recognizing antigens presented on MHC class I molecules?
- CD4+ Th1 cells
- CD4+ Th2 cells
- CD8+ cytotoxic T lymphocytes (CTLs) (Correct answer)
- Regulatory T cells (Tregs)
Correct answer: CD8+ cytotoxic T lymphocytes (CTLs)
CD8+ CTLs recognize viral peptides on MHC class I (expressed on all nucleated cells) and kill infected cells via perforin/granzyme and Fas-FasL pathways.
Question 5: Secretory IgA (sIgA) protects mucosal surfaces primarily by:
- Activating the complement cascade
- Neutralizing pathogens and preventing epithelial attachment (Correct answer)
- Enhancing phagocytosis via opsonization
- Triggering mast cell degranulation
Correct answer: Neutralizing pathogens and preventing epithelial attachment
sIgA neutralizes pathogens and blocks their attachment to mucosal epithelium without activating inflammation, providing non-inflammatory first-line mucosal defense.
Question 6: A fourfold or greater rise in antibody titer between acute and convalescent sera collected 2–4 weeks apart indicates:
- A prior resolved infection with residual immunity
- Active or recent infection with the tested pathogen (Correct answer)
- A nonspecific cross-reactive response
- Immunodeficiency with poor antibody production
Correct answer: Active or recent infection with the tested pathogen
A ≥4-fold rise in paired sera titer is the serological criterion for confirming active or recent infection, as seen in IgG class conversion and rising titers.
AP-Micro Immunology and Host Defense 2
Which cells present exogenous antigens via MHC class II molecules to activate CD4+ helper T cells?