For many other diseases it is suspected that such autoimmune reactions occur either as a cause of the disease or as a factor making symptoms more intense. Autoimmune diseases are thought to start when a virus with a protein coat invades certain cells. The cells already contain receptors on their surface that resemble the proteins in the viral coat.
Antibodies designed to attack the virus also attack the receptors on the cell, killing the cells.
Even after the virus infection is long gone, the circulating antibodies continue to destroy cells that have the receptor on their surface. Although it is not certain what initiates autoimmune diseases, there is some understanding of their development after initiation. For example, it is known that the autoimmune disease myasthenia gravis occurs because antibodies begin to attack a molecule used by nerves to direct the activity of muscles . In this disease the thymus trains part of the immune system to attack the acetylcholine receptor. If a person with myasthenia gravis is young enough, removal of the thymus may stop the disease, since only a small number of immune cells have been trained.