What is Autoimmunity?

Is the failure of an organism in recognizing its own constituent parts as self, which allows an immune response against its own cells and tissues. Any disease that results from such an aberrant immune response is termed an autoimmune disease.
Autoimmune diseases
Arise from an inappropriate immune response of the body against substances and tissues normally present in the body (autoimmunity). This may be restricted to certain organs(e.g. in autoimmune thyroiditis) or involve a particular tissue in different places (e.g. Goodpasture's disease which may affect the basement membrane in both the lung and the kidney). The treatment of autoimmune diseases is typically with immunosuppression-medication that decreases the immune response. A large number of autoimmune diseases are recognized.

(Ref: wikipedia.org)