Which T cell subset is activated when exogenous antigens are presented on MHC class II molecules?

• A. CD8+ cytotoxic T cells
• B. Regulatory T cells
• C. Gamma-delta T cells
• D. CD4+ helper T cells

Answer: (D)

Exogenous (extracellular) antigens are taken up by professional antigen-presenting cells and loaded onto MHC class II molecules. This presentation is recognized by CD4+ helper T cells, which, through their interaction with the MHC II-peptide complex, become activated. Activated helper T cells secrete cytokines that support B cell antibody production and enhance the functions of other immune cells like macrophages. In contrast, endogenous antigens are presented on MHC class I to CD8+ cytotoxic T cells, which are responsible for killing infected cells. Gamma-delta T cells and regulatory T cells have distinct roles and are not the primary responders to exogenous antigens presented on MHC class II.