Panel-reactive antibody
A panel-reactive antibody (PRA) is an antibody that is reactive against any of several known specific antigens in a test panel. A panel-reactive antibody test (PRA test) is an immunologic test routinely performed by clinical laboratories on the blood of people awaiting organ transplantation. 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 antigens, especially HLA system antigens. 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 HLA antigens, and so the PRA test panel constituents will differ from country to country.
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.