Autoimmunity, which is an abnormal immune response against self-antigens. The body cells (self) act as antigens and are termed as an autoantigen. The immune system is unable to differentiate between self and non-self tissue in case of auto-immune disorders. The immune system produces antibodies against tissues of the own body or self.
For example, in rheumatoid arthritis, a type of immunoglobulin IgM is diagnosed and it is due to autoimmunity of the synovial membrane. These antibodies destroy the synovial membrane, which is the autoantigen in this case. This leads to swollen, painful, and deformed joints.