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.
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.