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
The classification of the "A4" antigens was complicated. The "A4" subset evolved to become D-region antigens, which was a large cluster of genes that encoded MHC class II. Several renamings occurred. The D-region has 8 major coding loci that combine to form 3 different protein groups; DP, DQ, and DR. DRw antigens were the first to be split, a process made easy by the virtue of having an invariant alpha chain, but complicated by 4 beta chain loci (DRB1, DRB3, DRB4, and DRB5). Serotypes to DQ reacted with alpha and beta chains, or both of certain isoforms. The proper classification was greatly aided by gene sequencing and PCR. Classification and description of DP antigens is ongoing.