An autoimmune disease is the consequence of the hyperactivity of an immune system that goes beyond its role of defence against external agents and attack the body's own cells. White blood cells called lymphocytes are "abnormally" activated by external elements that disrupt the immune system of patients who have genetic predispositions. These activated lymphocytes then "attack" the tissues by acting, either directly like T lymphocytes or via autoantibodies produced by the B lymphocytes.

This tissue aggression will cause an inflammatory reaction that involves other immunity cells (polymorphonuclear and macrophages), which will amplify the lesions.

Autoimmune diseases are common, affecting a total of 3-5% of the population in countries such as France.

Schematically, there are two main types of autoimmune diseases:

• Systemic autoimmune diseases, i.e. diseases capable of affecting several organs, such as rheumatoid arthritis, systemic lupus erythematosus, systemic sclerosis (scleroderma), Sjögren syndrome and certain vasculitides. While rheumatoid arthritis is common (about 3 million patients in Europe), other conditions are rarer, such as lupus, which affects about 300,000 to 400,000 patients in Europe.

• Autoimmune diseases of organs (i.e. localised to a single organ) such as type I diabetes (involvement of the pancreas), thyroiditis (thyroid involvement), multiple sclerosis (damage to the brain and spinal cord), certain liver damage (liver diseases), or skin damage (bullous pemphigoid). Among these conditions, some are common such as type 1 diabetes or thyroiditis and others much rarer, such as liver disease. These diseases can exist in isolation or in combination, such as primary biliary cirrhosis with systemic scleroderma (Reynolds syndrome).