Autoimmune diseases are characterized by the immune system damaging a person's own tissues. These diseases are typically chronic, producing lifelong complications, and can ultimately be fatal. Myasthenia gravis is an example of an autoimmune disease. In myasthenia gravis, the body produces antibodies against the nicotinic acetylcholine receptor at the nerve-muscle junction. This disrupts proper nerve signaling, leading to the symptoms of muscle weakness commonly seen in myasthenia gravis. An autoantibody is an antibody that recognizes antigens on cells of the same individual that creates the antibodies. In healthy individuals, autoantibodies are of little consequence, but for some the immune system loses tolerance for self-antigens—substances within an organism that act as antigens, inducing an immune response—and autoantibodies mediate reactions that damage a person's own tissues. It is generally unclear for most autoimmune diseases what causes the loss of tolerance for self-antigens and the overproduction of autoantibodies.
There are many hypotheses that seek to describe a mechanism for how autoimmune diseases manifest, yet there is little experimental evidence. One hypothesis is that antigens released by pathogens during infection may closely resemble an antigen on a host's own cells. Antibodies build up against the pathogen during and following infection and then attack cells of the host's body postinfection. A primary example of this is demonstrated by group A streptococcal infection involving the upper respiratory tract, which can lead to the production of antibodies that later react with proteins on the heart valves, resulting in damage. In some parts of the world that do not have access to antibiotic treatment for streptococcal infection, six to eight weeks after the original upper respiratory infection, rheumatic fever can develop. Rheumatic fever is a febrile illness, or illness involving a fever response, affecting the joints, heart, skin, and brain.
There are a number of alleles associated with the development of autoimmune diseases. An allele is a version of a gene. People with family members that have an autoimmune disease are more likely to develop one themselves. There is also a higher prevalence of allergic hypersensitivities among people with family members that have autoimmune disorders. Many of the implicated alleles are for major histocompatibility complex genes that encode proteins used by the immune system, though the reason they are associated with higher autoimmune disease is yet unknown.
Autoimmune diseases in their various manifestations are relatively common. The particular symptoms and progression of disease is unique to each disease; there are few or no clinical similarities. Likewise, particular treatments and patient response to treatment varies by disease. Some of the more prevalent autoimmune diseases are systemic lupus erythematosus, rheumatoid arthritis, type I diabetes, multiple sclerosis, psoriasis, myasthenia gravis, and celiac disease.