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MHC molecules and lymphocytes: evolutionary perspective.

The classical polymorphic MHC molecules of class I or class II bind peptides derived from the processed cytosolic or endosomal antigens and export them to the cell surface for presentation to the T cell receptors (TcR) of CD8 or CD4 T lymphocytes, respectively. The classical MHC molecules are unstable when peptides are not bound. The MHC-peptide-TcR interactions constitute a molecular basis of thymic selection of the major streams of alpha beta and gamma delta T lymphocytes. The monomorphic MHC class I-like molecules (class Ib) bind peculiar peptides or nonpeptide antigens or can keep proper conformation even without antigenic peptides. They are recognized by the specialized subsets of nonconventional lymphocytes, mainly extrathymic gamma delta T or natural killer (NK) T lymphocytes. The most unorthodox T lymphocytes can see antigens directly without the participation of MHC or MHC-like molecules or can see MHC-like molecules not loaded with peptides. The conventional B2 lymphocytes are indirectly dependent on MHC-peptide-TcR interactions as they can bind the epitopes of native antigens via Ig surface receptors, to be activated they must present the processed antigens via the MHC class II molecules to the Th2 lymphocytes. In contrast, the B1 lymphocytes can be activated directly without cooperation with T cells via MHC molecules. It seems that both MHC molecules and lymphocyte antigen receptors arose by the expansion of Ig-superfamily genes at the early steps of vertebrate phylogeny. The nonconventional T lymphocytes (gamma delta T cells and NK T lymphocytes) and B1 cells which support innate immunity at the body surfaces or cavities as well as the MHC-like molecules might appear earlier, creating a proper microenvironment for development of the conventional T and B2 subsets of lymphocytes.