Which statement about MHC Class II is correct?

• A. MHC Class II molecules are on the surface of all nucleated cells
• B. MHC Class II molecules are on the surface of antigen-presenting cells and present exogenous peptides to helper T cells
• C. MHC Class II molecules present endogenous peptides to CD8+ T cells
• D. MHC Class II molecules are secreted by B cells

Answer: (B)

MHC Class II molecules are expressed on professional antigen-presenting cells and present exogenous peptides to CD4+ helper T cells. This pathway starts when extracellular proteins are taken up by antigen-presenting cells and broken down in endosomal compartments. The resulting peptides are then loaded onto MHC II molecules in these compartments and displayed on the cell surface for recognition by helper T cells, which coordinate the immune response, including activating B cells and supporting antibody production.

This differs from other statements: MHC Class II is not on the surface of all nucleated cells (that’s MHC Class I). MHC Class II presents exogenous, not endogenous, peptides. And MHC Class II molecules are membrane-bound on APCs, not secreted.