The immune system is able to identify nonself cells by aberrations in the MHC displayed on the plasma membrane. There are two groups of MHC molecules, and each group generates different markings on the plasma membrane:
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MHC‐I glycoproteins are produced by all body cells (except red blood cells). When a cell becomes cancerous or is invaded by a virus, unfamiliar proteins are synthesized in the cell. These proteins are endogenous antigens—that is, antigens produced inside the cell. Portions of these antigens are combined with MHC‐I glycoproteins and, when displayed on the plasma membrane, indicate a nonself cell.
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MHC‐II glycoproteins are produced only by antigen‐presenting cells (APCs)—mostly macrophages and B cells. APCs actively ingest exogenous antigens—antigens that originate outside the cell. Exogenous antigens include viruses, toxins, pollen, or bacteria that are circulating in the blood, lymph, or body fluids. APCs break down the antigens and incorporate pieces of them with MHC‐II glycoproteins. This aberrant display of MHC markers is recognized as nonself.