Antigen
In immunology, antigens (Ag) are structures (aka substances) specifically bound by antibodies (Ab) or a cell surface version of Ab ~ B cell antigen receptor (BCR). The term antigen originally described a structural molecule that binds specifically to an antibody only in the form of native antigen. It was expanded later to refer to any molecule or a linear molecular fragment after processing the native antigen that can be recognized by T-cell receptor (TCR). BCR and TCR are both highly variable antigen receptors diversified by somatic V(D)J recombination. Both T cells and B cells are cellular components of adaptive immunity. The Ag abbreviation stands for an antibody generator.
History and naming of human leukocyte antigens
Human leukocyte antigens (HLA) began as a list of antigens identified as a result of transplant rejection. The antigens were initially identified by categorizing and performing massive statistical analyses on interactions between blood types. This process is based upon the principle of serotypes. HLA are not typical antigens, like those found on surface of infectious agents. HLAs are "allo"antigens, they vary from individual to individual as a result of genetic differences.
An organ called the thymus is responsible for ensuring that any T-cells that attack self proteins are not allowed to live. In essence, every individual's immune system is tuned to the specific set of HLA and self proteins produced by that individual; where this goes awry is when tissues are transferred to another person. Since individuals almost always have different "banks" of HLAs, the immune system of the recipient recognizes the transplanted tissue as non-self and destroys the foreign tissue, leading to transplant rejection. It was through the realization of this that HLAs were discovered.