11.1.2—Antigens

An antigen is a molecular marker on a cell or pathogen surface that allows cell-to-cell recognition. The immune system uses the distinction between self and non-self antigens to decide whether an immune response is stimulated.

• Human body cells carry surface markers that identify them as part of the body.
• Bacteria and viruses carry their own surface markers; antigens can be found on bacterial cell walls or on virus surfaces.
• Some glycolipids and glycoproteins on the outer surface of cell-surface membranes act as antigens.

• Self antigen: produced by the organism’s own body cells; it normally does not stimulate an immune response.
• Non-self antigen: not produced by the organism’s own body cells, including antigens on pathogenic bacteria and viruses; it stimulates an immune response.

An antigen is a recognised surface marker, not necessarily the whole pathogen. Antigen recognition provides the signal for later specific immune responses; the detailed lymphocyte receptor matching, primary-response sequence and memory-cell mechanism belong to the following cards.