antigen

A protein that is recognised by the immune system (the body's natural defence system). The body does not usually react to antigens from its own cells, but should react vigorously to antigens from bacteria and other invaders. Sometimes the body will react to  its own antigens.  This is called auto-immune disease and can cause some types of kidney disease. The immune system reacts to antigens on a transplant and can lead to transplant rejection.