HLA system (human leucocyte antigen system) A series of four gene loci (A, B, C, and D) in humans that code for a group of glycoproteins, present on the surface of plasma membranes, that act as antigens and are important in determining the acceptance or rejection by the body of a tissue or organ transplant (see graft). These antigens are one group of the so-called histocompatibility proteins (see histocompatibility). Two individuals with identical HLA types are said to be histocompatible. Successful transplantation requires a minimum number of HLA differences between the donor's and recipient's tissues. See also major histocompatibility complex.

HLA system n. human leucocyte antigen system: a series of four gene families (termed A, B, C, and D) that code for polymorphic proteins expressed on the surface of most nucleated cells. Individuals inherit from each parent one gene (or set of genes) for each subdivision of the HLA system. If two individuals have identical HLA types, they are said to be histocompatible. Successful tissue transplantation requires a minimum number of HLA differences between donor and recipient tissues.