Which statement about MHC Class II is correct?

• A. On the surface of antigen-presenting cells and present exogenous peptides to helper T cells
• B. On all nucleated cells
• C. Present endogenous peptides to CD8+ T cells
• D. Secreted by B cells

Answer: (A)

MHC Class II molecules are specialized for presenting exogenous, or extracellular, peptide fragments to CD4+ helper T cells, and they are expressed on professional antigen-presenting cells such as dendritic cells, macrophages, and B cells. These cells take up extracellular proteins by endocytosis, process them in acidified endosomes, and load the resulting peptides onto MHC II molecules. The peptide–MHC II complex then travels to the cell surface to be recognized by CD4+ T cells, which helps coordinate the immune response, including helping B cells and activating macrophages.

This is distinct from MHC Class I, which is present on almost all nucleated cells and presents endogenous peptides to CD8+ cytotoxic T cells. Also, MHC II molecules are membrane-bound, not secreted; while B cells can present antigens via MHC II, the molecules themselves are not released into the surrounding environment.