What is an autoimmune disease and why does it occur? Why don't autoimmune diseases occur more commonly?

An autoimmune disease occurs when the immune system mounts an attack against its own cells. There are many factors that contribute to autoimmune diseases, with genetic, hormonal, and environmental factors all playing a role. Lymphocytes have receptors that recognize a vast  number of antigens, although individual lymphocytes will only identify one specific antigen. A lymphocyte that recognises a "self" antigen-an antigen on a cell that is a normal part of the body-it will not initiate an immune response. The lymphocytes learn which antigens are self early on fetal development. This is known as immune tolerance. If on the other hand the lymphocyte identifies a nonself antigen it will initiate an appropriate response. Autoimmune diseases occur when tolerance fails. Autoimmune diseases don't occur commonly because the immune system is very good at differentiating self from nonself. However, autoimmune diseases affect almost 10% of people in Western societies with women being far more commonly affected than men.