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T helper cells only recognize an antigen that is presented with class II major histocompatibility antigens whereas T cytotoxic cells usually only recognize a processed antigen in conjunction with class I major histocompatibility antigens.
MHC restrictionThe ability of T cells to recognise antigens when associated with the organism’s own MHC haplotype, providing a dual recognition system critical to T-cell function. (See Antigen-presenting cell, MHC.)
MHCR occurs in the thymus before a T cell becomes a functional antigen-specific cell in the peripheral immune system; the selection process operates on the α/β heterodimer of the T-cell receptor, assuring that the T cells will react with the product of the MHC, and not with self-antigens (i.e., the process is “self-tolerant”).