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.
Regular and irregular antibodies
"Regular antibodies" usually refer to the isohemagglutinins, directed against antigens of the ABO system. They appear in the first years of life. They are of the IgM type. It is not certain whether they are natural antibodies (being produced without any previous infection, vaccination, other foreign antigen exposure or passive immunization) or are caused by infection in early life.