The HLA system in blood transfusion.

The human leukocyte antigen (HLA) system, originally discovered as the result of a transfusion reaction, is now known to play a crucial role in many areas of clinical medicine. The main function of the HLA molecules is to present antigenic peptides to the immune system and in this way regulate the induction of immune responses. This is a highly regulated process which requires a close interaction between the HLA molecules, the antigenic peptide and the T cell receptor.HLA molecules are also known to be associated with a variety of autoimmune, non-autoimmune and infectious diseases and to restrict the antibody response to certain antigens and to vaccines. It is likely that the mechanism responsible for this restriction is the preferential presentation of antigen-derived peptides to T cells. Furthermore, HLA antigens, in contrast to most polymorphic molecules, have the ability to activate the immune system using two different pathways of T cell activation, the direct and indirect pathways. As a result of these features, HLA antigens and antibodies are responsible for some of the serious clinical complications of blood transfusion, and have an important influence on the outcome of solid organ and haemopoietic stem cell transplantation.