Power of Immune Responses

Many genes encode the molecules of the immune system. Class I HLA gene products constitute a functional receptor on most nucleated cells of the body and are responsible for superficial polymorphism in humans. In addition, the immune system uses the HLA molecules to differentiate self-cells and non-self cells. If a cell is infected by a virus, the Class I HLA molecules bring fragments of the virus to the surface of the cell so that the cell can be destroyed by cytotoxic CD8+T cells. Also, any infected cell loses Class I HLA molecules expressed on the surface of the cell, and it is rapidly attacked by NK cells, triggering apoptosis in it.
Class I and Class II HLA molecules take part in uploading antigenic epitopes on the groove of HLA molecules for presentation to lymphocytes. The groove of a single HLA molecule can accommodate a lot of different antigenic epitopes but cannot bind all epitopes in such a manner in order to reach a high concentration of antibodies and effector T-cells. Each individual is able to mount powerful immune responses only to certain antigens and weaker responses to others. That is why HLA genes may be defined as genes whose products control the power of adaptive immune responses.