In short, H2 (or HLA) molecules serve two linked purposes. They present peptides to a T cell so that a T cell can detect infections and other invaders and mount an immune response. And they are also the determinants by which one person’s cells are distinguished from another person’s cells, thereby defining the boundaries of an organism. Graft rejection (likely important for primitive organisms) and invader recognition (important for complex, multicellular organisms) are thus combined into a single system. Both functions repose in the T cell’s capacity to recognize the MHC peptide complex, or
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