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
In the early 1960s, some physicians began more aggressive attempts at organ transplantation. Knowing little about "compatibility factors", they attempted transplantation between humans and between non-humans and humans. Immunosuppressive drugs worked for a time, but transplanted organs would either always fail or the patients would die from infections. Patients received skin, white blood cell or kidney donations from other donors (called allografts, meaning 'of different genetics' grafts). If these allografts were rejected, it was found that the 'rejection' response was accompanied by an antibody mediated agglutination (biology) of red blood cells (See figure). The search for these cell surface antigens began. There are several processes by which antibodies can reduce function: