Antigenprocessing pathways

Class I MHC (HLA-A, HLA-B, and HLA-C in the human) contain mainly peptides derived from cytoplasmic proteins synthesized within the antigen-presenting cell itself. The peptides are produced within the cytoplasm by proteolytic degradation and transported into the endoplasmic reticulum via a specialized transporter of antigen peptide, where they bind within a nascent MHC chain during folding. The repertoire of peptides loaded is derived predominantly from normal cell proteins and in consequence is not seen by the immune system which is tolerant to self. However, in a virally infected or tumor cell, non-self or abnormal proteins may be synthesized by the cell's own machinery and these will also be presented on class I MHC and lead to a T-cell response, which is normally cytotoxic.

Class II MHC (HLA-DR, HLA-DP, and HLA-DQ in the human) contains predominantly peptides derived from membrane proteins of the antigen-presenting cell itself. Other peptides are derived from proteins taken up by endocytosis or phagocytosis. Therefore class II MHC will present any proteins derived from extracellular infectious agents (bacteria, parasites, or extracellular stages of viruses) which are taken up by antigen-presenting cells in the vicinity of infection.

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