immune system disease autoimmune disease An immune system disease that is an overactive immune response of the body against substances and tissues normally present in the body resulting from an abnormal functioning of the immune system that results in the production of antibodies or T cell directed against the host tissues. ICD9CM:720 disease_ontology DOID:417 MIM:109100 autoimmune disease UMLS_CUI:C0003089 Xref MGI. autoimmune hypersensitivity disease hypersensitivity reaction type II disease