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 donor 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. A PRA score greater than 6 is in danger, and over 20 requires a desensitization process, but its intensity varies. 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 has a different demographic prevalence of particular antigens, so the PRA test panel constituents differ from country to country.