Autoimmune diseases

Autoimmunity is the failure of an organism to recognize its own constituent parts (down to the sub-molecular levels) as "self", which results in an immune response against its own cells and tissues. Any disease that results from such an aberrant immune response is termed an autoimmune disease. Prominent examples include Coeliac disease, diabetes mellitus type 1 (IDDM), systemic lupus erythematosus (SLE), Sjögren's syndrome,  multiple sclerosis (MS), Hashimoto's thyroiditis, Graves' disease, idiopathic thrombocytopenic purpura, and rheumatoid arthritis (RA). See List of autoimmune diseases.
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Autoimmune diseases A group of diseases, like rheumatoid arthritis and systemic lupus erythematosus, in which immune cells turn on the body, [more]Autoimmune Diseases - Community and Resources

are illnesses which occur when the body tissues are attacked by its own immune system. The immune system is a complex organization within the body that is designed normally to "seek and destroy" invaders of the body, particularly infections. Patients with these diseases have unusual antibodies in their blood that target their own body tissues.