All in the superfamily: presentation of antigen receptors in the context of the major histocompatibility complex.
I propose that the antigen-induced internalization of antigen receptors of B or T lymphocytes serves to assure the interaction of those receptors, in intracellular compartments, with the major histocompatibility complex (MHC)-encoded molecules of the cells, which are themselves internalized constitutively. Antibody and T-cell receptors have conserved regions which promote the association of their unique amino acid sequences ("idiotypes") with MHC molecules as a consequence of exposure to the acidic environment of endocytic vesicles. If the antigen is internalized with the antigen receptor, it may associate with MHC molecules which it encounters after internalization if it has sequences permitting this association. Thus, these cells may express either "idiotype"-MHC or antigen-MHC complexes, or both, after the re-expression at the cell surface of the complexes which form intracellularly, permitting the recognition of these cells by T cells specific for these structures. This mechanism permits the organism to respond to antigens for which it has affinity, in a manner which does not depend on the structure of the antigen, by the use of its own receptors. These molecules co-evolve with the MHC molecules of the species in a way that preserves the possibility of their interaction.
D000818:Animals; D000938:Antigen-Presenting Cells / Q000276:immunology; D001402:B-Lymphocytes / Q000276:immunology*; D004705:Endocytosis; D006649:Histocompatibility Antigens / Q000276:immunology*; D000949:Histocompatibility Antigens Class II / Q000276:immunology; D006801:Humans; D008211:Lymphocyte Cooperation; D008264:Macrophages / Q000276:immunology; D008285:Major Histocompatibility Complex; D011947:Receptors, Antigen, B-Cell / Q000276:immunology*; D011948:Receptors, Antigen, T-Cell / Q000276:immunology*; D013601:T-Lymphocytes / Q000276:immunology*; D006377:T-Lymphocytes, Helper-Inducer / Q000276:immunology
D006649:Histocompatibility Antigens; D000949:Histocompatibility Antigens Class II; D011947:Receptors, Antigen, B-Cell; D011948:Receptors, Antigen, T-Cell