antigen

Meaning: An antigen is a molecule that an antibody, B-cell receptor or T-cell receptor recognizes and binds specifically. Most are proteins or polysaccharides from pathogens, but allergens, transplanted tissue and sometimes self molecules also act as antigens. The exact site that is bound is the epitope, and one antigen can carry several epitopes. B cells bind intact antigen with membrane-bound antibody. T cells recognize antigen fragments displayed on MHC molecules: helper T cells on MHC II, cytotoxic T cells on MHC I. Binding, together with costimulation, selects the matching lymphocyte, which divides into effector cells and memory cells (clonal selection). Memory cells produce a faster, larger secondary response on re-exposure.

On the MCAT: Expect passages on vaccines, blood typing, transplant rejection and graphs of primary versus secondary antibody response. A vaccine supplies an antigen without causing the disease, so memory cells form. In ABO typing, type A blood has A antigens on red cells and anti-B antibodies in plasma; type O has neither A nor B antigens. Rh-positive cells carry the Rh antigen.

Common confusion: Antigen and antibody are not interchangeable. The antigen is the target; the antibody (immunoglobulin) is the Y-shaped protein secreted by plasma cells that binds it. Also, antibodies bind an epitope, not the whole antigen.