The immune system is the body's defense system. It protects the body from outside threats such as bacteria and viruses that cause infections. When the body comes under attack, immune cells and immune proteins attack the intruding bacteria or virus. This will hopefully defeat the illness before it can become an issue in the body. Typically, an observed elevation in immune cells and proteins is an indication that the body is fighting an infection. Several studies have described abnormal fluctuations in the immune system that vary according to the state of bipolar illness. For example, T- and B-lymphocytes, IgG (immunoglobulin G), and IgM (immunoglobulin M) are increased in patients with bipolar disorder. This observation would lead to the conclusion that the bodies of individuals with bipolar disorder are fighting an infection. But it is unclear whether these findings are the cause or result of natural body responses to mood peaks and dips or the use of medications. In addition to measuring immune system components in the study of bipolar disorders, infections have also been investigated for their contribution to bipolar symptoms.
Another aspect of the immune system that is involved in bipolar disorders is autoimmunity. Autoimmune diseases result when the immune system becomes confused. It can no longer distinguish between the body's own tissues and those which belong to an outside threat. As a consequence of this confusion, the body starts attacking itself. For example, in rheumatoid arthritis, the immune system attacks the joints, causing a disabling and painful inflammatory condition. The disease leads to destruction of the joints as well as tissues throughout the body including the skin, blood vessels, heart, lungs, and muscles.
There is no evidence linking bipolar disorders directly to an autoimmune condition. However, autoimmunity against the thyroid gland is well established. Given that mood changes are associated with thyroid hormone imbalances, it is conceivable that thyroid autoimmunity is connected to bipolar disorders. However, as is the case with infectious diseases, there is no clear evidence at this time that autoimmunity is necessarily involved in bipolar disorders.