Which statement about MHC class II molecules is true?

Which statement about MHC class II molecules is true?

• A. They present endogenous peptides to CD8+ T cells.
• B. They present exogenous peptides to CD4+ helper T cells.
• C. They are expressed on all nucleated cells.
• D. They do not participate in antigen presentation.

Answer: (B)

MHC class II molecules specialize in presenting peptides derived from extracellular proteins to CD4+ helper T cells. This pathway starts when proteins outside the cell are taken up by professional antigen-presenting cells (such as dendritic cells, macrophages, and B cells) and degraded in endosomal/lysosomal compartments. The MHC II molecule, initially blocked by the invariant chain in the ER, is loaded with peptides inside these acidic compartments with the help of accessory molecules like HLA-DM, and then the peptide-MHC II complex is transported to the cell surface for recognition by CD4+ T cells. This interaction activates helper T cells, which coordinate the immune response, including helping B cells produce antibodies and activating macrophages. Endogenous peptides presented to CD8+ T cells are presented by MHC class I, not II. MHC II is not expressed on all nucleated cells—primarily it’s on professional APCs, though some cells can express it under certain conditions. And they do participate in antigen presentation, so stating they do not would be incorrect.

MHC class II molecules specialize in presenting peptides derived from extracellular proteins to CD4+ helper T cells. This pathway starts when proteins outside the cell are taken up by professional antigen-presenting cells (such as dendritic cells, macrophages, and B cells) and degraded in endosomal/lysosomal compartments. The MHC II molecule, initially blocked by the invariant chain in the ER, is loaded with peptides inside these acidic compartments with the help of accessory molecules like HLA-DM, and then the peptide-MHC II complex is transported to the cell surface for recognition by CD4+ T cells. This interaction activates helper T cells, which coordinate the immune response, including helping B cells produce antibodies and activating macrophages.

Endogenous peptides presented to CD8+ T cells are presented by MHC class I, not II. MHC II is not expressed on all nucleated cells—primarily it’s on professional APCs, though some cells can express it under certain conditions. And they do participate in antigen presentation, so stating they do not would be incorrect.