Human leukocyte antigens (HLA) are specialized proteins present on the surface of all cells in the body except red blood cells. The HLA genes that individuals inherit are responsible for the HLA antigens present on their cells. HLA testing identifies the major HLA antigens that are present on the surface of an individual's cells and the antibodies to HLA antigens as well as the genes that are responsible for HLA antigens, primarily to match transplant donors and recipients. In humans, the HLA genes are located in region of chromosome 6 called the major histocompatibility complex (MHC). The MHC plays an essential role in the management of the immune system. It helps the body's immune system distinguish which cells are "self" and which are "foreign" or "non-self." Any cells that are recognized as "non-self" can trigger an immune response, including the production of antibodies. HLA antibody testing is performed on transplant recipients to determine if they have antibodies that would target the HLA antigens on the donated organ or tissue. HLA antibody testing is also performed on platelet transfusion recipients to determine if they have any antibodies that would target the donor's platelets and prevent a good response to the transfusion. For hematopoietic stem cell (HSC) transplant, the HLA genes of the donor and recipient must be the same or match as closely as possible for transplantation to be successful and to lessen the chance of developing graft-versus-host disease (GVHD). In solid organ transplants, such as kidney, heart, or lung transplants, ABO blood group compatibility is critical. After matching ABO types, it is also beneficial to match the HLA antigens between the donor and recipient. Unlike ABO matching, HLA typing mismatches are less critical as long as the recipient has not developed HLA antibodies directed against donor's antigens. Various drugs may be administered to help suppress the recipient's immune system in order to minimize organ rejection. Finding a donor who is compatible with an intended recipient may sometimes be difficult. Part of the reason is because each HLA gene can have numerous possible forms or variations (alleles). This characteristic is referred to as polymorphic. In addition, there are more than 200 genes that make up the large HLA gene family located on chromosome 6. Considering the many different possible combinations and numerous types of HLA alleles, it can be a challenge to find a suitable donor, especially if the recipient has pre-formed HLA antibodies. Nevertheless, HLA genes are located close together and inherited together as groups known as haplotypes; thus, a child inherits one haplotype from each parent. Because of this, there is a greater chance that family members will have the same group of HLA alleles compared to non-related potential donors. Often, a recipient's parents, children, or siblings may serve as the best transplant matches.