Major Histocompatibility Complex - T Lymphocyte Recognition Restrictions

T Lymphocyte Recognition Restrictions

In their development in the thymus, T lymphocytes are selected to recognize MHC molecules of the host but not recognize other self antigens. Following selection each T lymphocyte shows dual specificity: The T cell receptor (TCR) recognizes self MHC but only non-self antigens.

MHC restriction occurs during lymphocyte development in the thymus through a process known as positive selection. T cells that do not receive a positive survival signal — mediated mainly by thymic epithelial cells presenting self peptides bound to MHC molecules — to their TCR undergo apoptosis. Positive selection ensures that mature T cells can functionally recognize MHC molecules in the periphery (i.e. elsewhere in the body).

The TCRs of T lymphocytes recognise only sequential epitopes, also called linear epitopes, of only peptides and only if coupled within an MHC molecule. (Antibody molecules secreted by activated B cells, on the other hand, ligate diverse epitopes—peptide, lipid, carbohydrate, and nucleic acid—and recognize conformational epitopes, which have 3D structure.)