Antigen
Also, an antigen is a molecule that binds to Ag-specific receptors, but cannot necessarily induce an immune response in the body by itself. Antigens are usually proteins, peptides (amino acid chains) and polysaccharides (chains of monosaccharides/simple sugars) but lipids and nucleic acids become antigens only when combined with proteins and polysaccharides. In general, saccharides and lipids (as opposed to peptides) qualify as antigens but not as immunogens since they cannot elicit an immune response on their own. Furthermore, for a peptide to induce an immune response (activation of T-cells by antigen-presenting cells) it must be a large enough size, since peptides too small will also not elicit an immune response.
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.