Which statement about MHC class II molecules is correct?

• A. They are found on all nucleated cells and present endogenous antigens to CD8+ T cells
• B. They are found on antigen-presenting cells and present exogenous antigens to CD4+ T cells
• C. They are found on red blood cells and present antigens to B cells
• D. They are found only on B cells and present antigens to NK cells

Answer: (C)

MHC class II molecules present external antigens to CD4+ T helper cells, and they’re found mainly on professional antigen-presenting cells such as dendritic cells, macrophages, and B cells. The process starts when extracellular proteins are taken up by endocytosis or phagocytosis, then broken down into peptides inside endosomes/lysosomes. These peptides are loaded onto MHC class II molecules in acidified compartments and transported to the cell surface. The peptide-MHC II complex is recognized by the T cell receptor on CD4+ T helper cells, with CD4 aiding the interaction. This activation helps coordinate antibody production by B cells and enhances macrophage responses. In contrast, MHC class I molecules are present on nearly all nucleated cells and present endogenous peptides to CD8+ cytotoxic T cells, not exogenous antigens. Red blood cells do not express MHC class II and do not present antigens to B cells.