The HLA System in Transfusion Medicine and Transplantation

The human leukocyte antigen (HLA) system has been well known as transplantation antigens, and clinical HLA testing has evolved in support of transplantation, but the primary biological role of HLA molecules is in the regulation of immune response. HLA alloimmunization can be induced by blood transfusions, pregnancies, or failed organ transplants. Actual DNA sequencing of amplified products of multiple HLA loci, called sequence‐based typing, has been increasingly used as clinical HLA typing to select HLA‐allele matched donors in support of unrelated donor stem cell transplantation. Minor histocompatibility antigens are processed peptides derived from normal cellular proteins that associate with self‐HLA molecules. The HLA molecules are major transplantation antigens. The HLA system can cause adverse immunologic effects in transfusion therapy. These effects are primarily caused by donor “passenger” leukocytes contained in the cellular blood components.