Panel-reactive antibody

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A panel-reactive antibody (PRA) is a group of antibodies in a test serum that are reactive against any of several known specific antigens in a panel of test cells or purified HLA antigens from cells. A panel-reactive antibody test (PRA test) is an immunologic test routinely performed by clinical laboratories on the blood of people awaiting organ transplantation. In this test recipient cells confort to random cells of doner population and estimation risk of acute rejection. The PRA score is expressed as a percentage between 0% and 100%. It represents the proportion of the population to which the person being tested will react via pre-existing antibodies against human cell surface antigens, which include human leukocyte antigen|HLA] and other polymorphic antigen systems. PRA more than 6 is in danger and over 20 needs desensitization process, but its intensity vary. These antibodies target the surface antigens of target cells, such as HLAs. In other words, it is a test of the degree of alloimmunity in a graft recipient and thus a test that quantifies the risk of transplant rejection. Each population will have a different demographic prevalence of particular antigens, and so the PRA test panel constituents will differ from country to country.

Traditional PRA test is performed using a panel of lymphocytes from potential donor population. Since late 1990s', purified HLA antigen panel has been used to replace cell panel for PRA test, based on the assumption that HLA is the major target antigen system of alloantibody reaction. However, non-HLA antibody effect on PRA test has been ignored. (ref:Human leukocyte antigen antibodies for monitoring transplant patients. Surg Today. 2005;35(8):605-12.)

A high PRA value usually means that the individual is primed to react immunologically against a large proportion of the population. Individuals with a high PRA value are often termed "sensitized", which indicates that they have been exposed to "foreign" (or "non-self") proteins in the past and have developed antibodies to them. These antibodies develop following previous transplants, blood transfusions and pregnancy. Transplanting organs into recipients who are "sensitized" to the organs significantly increases the risk of rejection, resulting in higher immunosuppressant requirement and shorter transplant survival. People with high PRA scores therefore spend longer waiting for an organ to which they have no pre-existing antibodies.

Extensive efforts have been made to identify treatment regimes to reduce PRA in sensitized transplant candidates. In certain circumstances, plasma exchange, intravenous immunoglobulin, rituximab and other "antibody-directed" immune therapies may be employed, but this is an area in which active investigation continues.

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