The functions performed by the complex are: binding of peptide fragments from invading pathogens and then displaying these peptide fragments on the cell surface so that it should be recognized by T-cells fro further degradation.
MHC has got two unique properties, which make it difficult for any pathogen to evade immune responses in this way. These are:i) the MHC is polygenic: there are different varieties of MHC class I and MHC class II genes, hence every individual has got a set of MHC molecules with different ranges of peptide-binding specificities.ii) the MHC is highly polymorphic; there are multiple variants of each gene within the population as a whole. The MHC genes are, in fact, the most polymorphic genes known (Janeway 2001).
It has become an important and more frequent clinical practice to replace diseased organs through transplantation. The response of the body to tissues with nucleated cells is more severe, as compared to blood transfusion containing anucleated RBCs, and it is the response of T-cells to highly polymorphic MHC molecules. This can be achieved by matching of the MHC type between donor and recipient that is not possible to 100%. (Janeway 2001)
Previously rejected graft from a donor decreases the probability of acceptance of the second graft from the same donor to the same recipient and this response even earlier than the first set reject when body rejects it within a week time. This is again mediated by CD8 T cells, by CD4 T cells, or by both and at times even antibodies also become a part of this team (Janeway 2001).
Matching donor and recipient at the MHC
When donor and recipient differ at the MHC, the immune response, which is known as an alloreactive response as it is directed against antigens (alloantigens) that differ between members of the same species, is directed at the nonself allogeneic MHC molecule or molecules present on the graft.
Most tissues contain predominantly MHC class I antigens. Once a graft is rejected due to presence of a particular MHC type, the second graft with the same nonself MHC molecule will also be rejected in a second-set response in a rapid manner. This is because of the frequency of T cells specific for any nonself MHC molecule that is very high, making differences at MHC loci the most potent trigger of the rejection of initial grafts (Janeway 2001).
In human, HLA matching significantly improves the success rate of clinical organ transplantation but even it cannot in itself prevent rejection reactions. There are two main reasons behind this phenomenon: a) HLA typing is imprecise, owing to the polymorphism and complexity of the