what is an antigen?
An antigen is a substance that enters the body and stimulates the production of an antibody to fight what the immune system perceives as an invader. Antigens can be toxins, bacteria, foreign blood cells and the cells of transplanted organs. Other antigens, called autoantigens, or self-antigens, are usually normal proteins in the body that are targeted by the immune system of lupus patients as foreign bodies. 1  In other words, normal tolerance of these autoantigens has been lost in lupus patients, mainly to genetic and environmental factors.