MHC Class II molecules present antigens to helper T cells from which source?

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Multiple Choice

MHC Class II molecules present antigens to helper T cells from which source?

Explanation:
MHC class II presentation specializes in peptides from outside the cell. Antigen-presenting cells such as dendritic cells, macrophages, and B cells take up exogenous proteins from pathogens or the environment, process them in endosomes/ lysosomes, and load the resulting peptides onto MHC II molecules. These peptide–MHC II complexes then travel to the cell surface to be recognized by CD4+ helper T cells, coordinating the immune response. This is why the best choice emphasizes exogenous antigens carried by antigen-presenting cells—the source is external and the presentation requires the APC to have taken up and processed the antigen. Endogenous antigens are presented on MHC I to CD8+ cytotoxic T cells, which is a different pathway, so that option doesn’t fit MHC II.

MHC class II presentation specializes in peptides from outside the cell. Antigen-presenting cells such as dendritic cells, macrophages, and B cells take up exogenous proteins from pathogens or the environment, process them in endosomes/ lysosomes, and load the resulting peptides onto MHC II molecules. These peptide–MHC II complexes then travel to the cell surface to be recognized by CD4+ helper T cells, coordinating the immune response. This is why the best choice emphasizes exogenous antigens carried by antigen-presenting cells—the source is external and the presentation requires the APC to have taken up and processed the antigen. Endogenous antigens are presented on MHC I to CD8+ cytotoxic T cells, which is a different pathway, so that option doesn’t fit MHC II.

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