Antigen
Antigens are "targeted" by antibodies. Each antibody is specifically produced by the immune system to match an antigen after cells in the immune system come into "contact" with it; this allows a precise identification or matching of the antigen and the initiation of a tailored response. The antibody is said to "match" the antigen in the sense that it can bind to it due to an adaptation in a region of the antibody; because of this, many different antibodies are produced, each able to bind a different antigen while sharing the same basic structure. In most cases, an adapted antibody can only react to and bind one specific antigen; in some instances, however, antibodies may cross-react and bind more than one antigen.
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.