T-cell development — If MHC class II is not expressed in the thymus, which function would NOT be reduced (i.e., is the exception)?

Introduction:T-cell positive selection in the thymus depends on cortical epithelial cell expression of MHC molecules. Class II MHC is required to select CD4 T cells, while class I MHC is needed for CD8 T cells. This question asks which immune function would NOT be reduced if thymic MHC class II were absent.

Given Data / Assumptions:

• MHC class II expression in thymus is essential for generating CD4 T cells.
• CD4 T cells provide help to B cells (class switching to IgG) and activate macrophages (Th1) for intracellular/vesicular pathogens.
• CD8 T cells are selected on MHC class I, not class II.
• Alternative complement activation is innate and MHC-independent.

Concept / Approach:Predict consequences of lacking CD4 T cells: decreased Th-dependent macrophage activation and impaired T-dependent antibody responses (e.g., IgG). Identify which listed function relies on CD8 or innate pathways and therefore would not necessarily be reduced.

Step-by-Step Solution:

Verification / Alternative check:Patients with bare lymphocyte syndrome type II (MHC II deficiency) show markedly decreased CD4 counts and impaired humoral responses, while CD8 T cells can be near normal since MHC I is preserved.

Why Other Options Are Wrong:

Common Pitfalls:Assuming all T-cell functions decline; failing to separate CD4-dependent functions from CD8-dependent or innate pathways.

Final Answer:CD8 T cell-mediated cytotoxicity would NOT be reduced.