MHC class II molecules are primarily found on which cells and present antigens to which T cells?

• A. All nucleated cells to CD8+ T cells
• B. APCs to CD4+ T cells
• C. Platelets to NK cells
• D. Red blood cells to B cells

Answer: (B)

MHC class II molecules are used to present extracellular (exogenous) antigen fragments to CD4+ helper T cells and are expressed mainly on professional antigen-presenting cells such as dendritic cells, macrophages, and B cells. The peptide-MHC II complex on these cells activates CD4+ T cells, which then assist other parts of the immune response (e.g., helping B cells produce antibodies and activating macrophages). This is why the correct pairing is APCs presenting to CD4+ T cells. In contrast, MHC class I molecules are found on nearly all nucleated cells and present to CD8+ cytotoxic T cells; red blood cells don’t present antigens via MHC II, and platelets aren’t the typical APCs in this context.