A panel-reactive antibody (PRA) is a group of antibodies in a test serum that are reactive against any of several known specific

antigen In immunology, an antigen (Ag) is a molecule or molecular structure or any foreign particulate matter or a pollen grain that can bind to a specific antibody or T-cell receptor. The presence of antigens in the body may trigger an immune respons ...

s in a panel of test cells or purified HLA antigens from cells. It is an immunologic test routinely performed by clinical laboratories on the

blood Blood is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the cir ...

of people awaiting

organ transplantation Organ transplantation is a medical procedure in which an organ is removed from one body and placed in the body of a recipient, to replace a damaged or missing organ. The donor and recipient may be at the same location, or organs may be transpor ...

. In this test recipient cells are exposed 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 Alloimmunity (sometimes called isoimmunity) is an immune response to nonself antigens from members of the same species, which are called alloantigens or isoantigens. Two major types of alloantigens are blood group antigens and histocompatibility ...

in a graft recipient and thus a test that quantifies the risk of

transplant rejection Transplant rejection occurs when transplanted tissue is rejected by the recipient's immune system, which destroys the transplanted tissue. Transplant rejection can be lessened by determining the molecular similitude between donor and recipient ...

. Each population has a different demographic prevalence of particular antigens, so the PRA test panel constituents differ from country to country. A traditional PRA test is performed using a panel of lymphocytes from potential donor population. Since late 1990s, a purified HLA antigen panel has been used to replace a cell panel for the PRA test, based on the assumption that HLA is the major target antigen system of alloantibody reaction. However, the non-HLA antibody effect on the 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 wait longer 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 Rituximab, sold under the brand name Rituxan among others, is a monoclonal antibody medication used to treat certain autoimmune diseases and types of cancer. It is used for non-Hodgkin lymphoma, chronic lymphocytic leukemia (in non-geriatric pa ...

and other "antibody-directed" immune therapies may be employed, but this is an area in which active investigation continues.

References

{{reflist Immune system