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
The Hu-1 antigens were renamed the Human-lymphoid (HL) allo-antigens (HL-As). Allo-antigen comes from the observation that a tolerated protein in the donor becomes antigenic in the recipient. This can be compared with an autoantigen, in which a person develops antibodies to one or more of their own proteins. This also suggested the donor and recipient have a different genetic makeup for these antigens. The "LA" group thereafter was composed of HL-A1, A2, A3, A5, A6, A7, A8, A9, A10, A11, A12, A13, A14 and A15 until further divisions and renaming were necessary. Some of the antigens above, for example HL-A1, are similar to HLA-A1, as they are the same serotype. Some of the above, like A5, are not mentioned within the last few years, as they have been renamed.