When the immune system is functioning normally, the body develops a state of tolerance in response to its own proteins. In some cases, however, a state of autoimmunity may develop, in which the immune system recognizes and reacts to one or more of the body’s proteins. When this happens, the immune reaction that develops can have a serious impact on health.
The immune system has evolved into a highly specialized response that recognizes and destroys foreign proteins and organisms. In this regard, the immune system distinguishes only between “self” and “non-self.” Proteins that are produced by the body are self and trigger a state of immunological tolerance; proteins from viruses, bacteria, parasites, plants, animals, and other humans, are non-self and can potentially trigger an immune response.
Immunological tolerance is a state in which the immune response is actively suppressed towards self proteins. In some situations, however, the body can develop an immune response, rather than tolerance, to one or more self proteins. This self-directed immune response is called autoimmunity.
The most well-known autoimmune disorders include type 1 diabetes and rheumatoid arthritis, as well as multiple sclerosis and lupus. In each case, autoimmune disease symptoms are caused when the immune system attacks self proteins. The resulting immune response can cause tissue destruction, chronic inflammation, and other debilitating symptoms.
In the case of type 1 diabetes, for example, immune cells destroy the insulin-producing cells of the pancreas, leading to the need for an external source of insulin. In contrast, rheumatoid arthritis is the result of a cell-based immune response to joint tissue which causes chronic pain and inflammation. Systemic lupus occurs when the body produces autoimmune antibodies that react to a type of protein found in almost all cells in the body. This form of the disease can be particularly dangerous, with the ability to affect organs, joints, muscle, and blood.
While the general causes of autoimmunity are unknown, a range of theories attempt to account for patterns of autoimmune disease development in populations. One theory builds on the fact that women are more likely to develop autoimmunity than men, and further that pregnancy increases a woman’s risk of developing an autoimmune disorder. According to this theory, this risk increases because women are exposed to fetal proteins across the placental barrier during pregnancy, which might disrupt self protein tolerance.
Another theory attempts to explain why the incidence of autoimmune disorders has increased in many Western countries over the last several decades. The hygiene hypothesis explains that an increased focus on hygiene has led to reduced exposure to non-self proteins, and reduced opportunity for the immune system to "learn" how to distinguish between self and non-self. There is some evidence for this theory in the fact that autoimmunity is much more prevalent in the Western world than in countries where one or more infectious diseases are endemic to the population.