Which MHC class presents extracellular antigens to CD4+ T cells?

Which MHC class presents extracellular antigens to CD4+ T cells?

• A. MHC class I
• B. MHC class II
• C. MHC class III
• D. CD1

Answer: (B)

MHC class II molecules are the ones that present peptide antigens from outside the cell to CD4+ helper T cells. Antigens that are extracellular are captured by professional antigen-presenting cells such as dendritic cells, macrophages, and B cells, then processed in endosomal-lysosomal compartments. The resulting peptide fragments load onto MHC II in these acidic vesicles with the help of escort molecules like the invariant chain and HLA-DM, and the peptide-MHC II complex is displayed on the cell surface. When a CD4+ T cell recognizes this complex via its T-cell receptor, it becomes activated and can orchestrate coordinated immune responses, including helping B cells produce antibodies and activating macrophages. MHC class I presents endogenous, intracellular peptides to CD8+ cytotoxic T cells, not CD4+ helper T cells. CD1 presents lipid antigens to a subset of T cells, not typical peptide presentation by MHC. MHC class III encodes components of the complement system and cytokines, not peptide presentation.

MHC class II molecules are the ones that present peptide antigens from outside the cell to CD4+ helper T cells. Antigens that are extracellular are captured by professional antigen-presenting cells such as dendritic cells, macrophages, and B cells, then processed in endosomal-lysosomal compartments. The resulting peptide fragments load onto MHC II in these acidic vesicles with the help of escort molecules like the invariant chain and HLA-DM, and the peptide-MHC II complex is displayed on the cell surface. When a CD4+ T cell recognizes this complex via its T-cell receptor, it becomes activated and can orchestrate coordinated immune responses, including helping B cells produce antibodies and activating macrophages.

MHC class I presents endogenous, intracellular peptides to CD8+ cytotoxic T cells, not CD4+ helper T cells. CD1 presents lipid antigens to a subset of T cells, not typical peptide presentation by MHC. MHC class III encodes components of the complement system and cytokines, not peptide presentation.