Immunology and the Immune System Practice Test

MHC class II molecules present peptide antigens to CD4+ helper T cells. These molecules are on professional antigen-presenting cells (like dendritic cells, macrophages, and B cells) and display extracellular-derived peptides loaded in endosomes after uptake. The CD4 co-receptor on helper T cells recognizes the peptide-MHC II complex, triggering activation and cytokine signals that coordinate the immune response, such as helping B cells and macrophages. In contrast, MHC class I presents to CD8+ cytotoxic T cells, and NK cells do not rely on MHC II for antigen recognition. B cells can present to CD4+ T cells via MHC II, but the T cells that specifically recognize MHC II-peptide complexes are CD4+ helper T cells.