The human leukocyte antigen (HLA) is not a single antigen, but is rather a group of proteins that are located on the surface of white blood cells. These proteins have a pivotal role in the body's immune response to foreign material. Because the HLA is a chemical tag that distinguishes "self" from "non-self," the antigen is important in the rejection of transplanted tissue and in the development of certain diseases (e.g., insulin-dependent diabetes).

The HLA is the human version of a complex that is known as the major histocompatibility complex. Similar complexes exist in other species. Indeed much of the early knowledge of the antigen complex came from work on mice in the early decades of the twentieth century. Research on human blood cells in the 1950s identified three genes associated with the HLA (HLA-A, HLA-B, HLA-C). In the 1970s, another...