The immune system presents antigenic peptides loaded onto MHC Class I and Class II molecules through distinct pathways. MHC Class I loads endogenous peptides in the cytosol, while Class II loads exogenous peptides in endosomes/phagosomes. Both pathways result in efficient activation of T cells and appropriate immune responses. (Word count: 50)

The high polymorphism of MHC genes benefits species by enhancing immune responses and genetic diversity, but poses challenges in organ transplantation due to histocompatibility matching issues and the risk of graft rejection. This requires a delicate balance between evolutionary advantages and clinical challenges.

Classical MHC class I and class II molecules play distinct roles in antigen presentation to T cells. MHC class I consists of a single polypeptide chain with β2-microglobulin, presenting endogenous peptides to CD8+ cytotoxic T cells, while MHC class II has two polypeptide chains, presenting exogenous peptides to CD4+ helper T cells. Both are vital for adaptive immunity.