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
In this arrangement there were cells that were 'blank' or had new specificities, these new antigens were called "W" antigens, and as they were reassigned to new groups, for example "A" serotypes, they became Aw or Bw antigens. It was found that some antigens that behaved like A and B antigens but could be excluded based on '2-type max' exclusion. Thus a new group, "C" was created. Classification of C antigens is still ongoing, and they have retained the name Cw as many serotypes have not been developed.