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Saturday, September 24, 2016

TRANSPLANTATION IMMUNO-GENETICS OF HLA


HLA testing and organ transplant

HLA testing and organ transplant

Major Histocompatibility Complex




Embedded in the membrane of every human cell is an array of molecules many of them glycoprotein that are recognized by the immune system of another human as antigens.

Normally, the body's immune system does not respond to its own membrane antigens because they are self. Transplanted foreign cells, however, carry nonself membrane antigens that can be recognized, and following the recognition, the cells carrying those antigens can be destroyed.

Thus, for a successful transplant, there must be minimal differences between the antigens of the recipient and the donor, ie, the individuals must be histocompatible. Ultimately, of course, the search for compatibility requires an understanding of genetic control of histocompatibility antigens.

A small segment of the chromosome is referred to as the major histocompatibility complex (MHC) and encodes within it the strongest histocompatibility or transplantation antigens. This complex of deoxyribonucleic acid (DNA) comprises approximately 1/1000 of all genetic material.

These are the antigens that if recognized as foreign in cells of an allograft, lead to rapid destruction of that allograft. Various aspects of the major histocompatibility complex in humans referred to as HLA. Besides these very strong transplantation antigens, a number of other immunologically related phenomena are controlled by or at least influenced by genes located in the MCH, eg, immune responsiveness (Ir), susceptibility to some diseases various components of the complement system and cell-mediated responses against virally infected autologous or syngeneic cells.

The evidence for immune responsiveness genes in humans is still fragmentary, although the MHC includes Ir genes in several other species, for example, mouse, guinea pigs, rat, and rhesus monkey. 

This article discusses the detection of histocompatibility antigens in humans and how they are controlled by MHC genes. Grafts may be rejected if incompatibility loci that are not encoded by the MHC referred to as minor histocompatibility loci.  

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