The major histocompatability complex (MHC) codes for HLA antigens present on white cells and body tissues (Figure 2-5). It consists of several subloci, each of which has multiple alleles. Class I antigens occur at the A, B, and C subloci and are present on all nucleated body cells. Class II antigens occur at the D sublocus which is further divided into DP, DQ, and DR/OCT) and are present mainly on cells of the immune system (e.g., macrophages, T and B cells). HLA antigens and antibodies have relevance for blood transfusion because the antibodies are a major cause of febrile transfusion reactions. They can be used in paternity disputes because of their polymorphism. As well, they have relevance to transplantation because they are present on body tissues. We now know that the MHC plays a major role in immunity through MHC restriction.
MHC restriction means that different T cells are restricted to either Class I or Class II MHC antigens (Figure 2-6). Cytotoxic T cells are restricted to Class I antigens present on nucleated body cells, thus play a role in protecting against virus-infected cells or cancerous cells. Helper CD4 cells are restricted to Class II MHC antigens present on immune cells, thus play a role in increasing the humoral immune response.
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| MHC Restriction |
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