A defect in immunological tolerance may either secure spontaneously to be induced by some exogenous factor such as virus infection, often in a genetically predisposed individual.
So, autoimmunity is an abnormal immune response against self-antigens. Autoimmune diseases are of two types—organ specific and non-organ specific. Organ Specific autoimmune diseases: Hashimoto’s thyroiditis, primary myxoedema, thyrotoxicosis, pernicious anaemia, autoimmune Addison’s disease, Good Pasteur’s syndrome, Myasthenia gravis, sympathetic ophthalmia, chronic active hepatitis, primary biliary cirrhosis. Non-organ specific autoimmune diseases: Rheumatoid arthritis, dermatomyositis, systemic sclerosis, systemic lupus erythematosus (SLF).
Virtually any component of immune system, specific or non specific, can be absent or abnormal, the consequent immunodeficiency states vary in severity from trivial to fatal. There are many genetically determined conditions, but immunodeficiency can also result from acquired diseases. SCID (Severe Combined Immuno Deficiency) is a congenital immunodeficiency disorder in which both B-and T- lymphocytes are not formed and the patient dies of even minor infection. AIDS is an acquired immune deficiency syndrome which is characterized by reduction in the number of helper T-lymphocytes because of infection of HIV (human immunodeficiency virus).
Hypersensitivity is an exaggerated (intensified) immune response that causes damage to the individual. Immediate hypersensitivity is mediated by antibody or immune complexes, and delayed- type hypersensitivity is mediated by TDTH cells. Hypersensitivity may result from the induction of an inappropriate pattern of immunity. Hypersensitivity may also occur when the immune response to an antigen such as a virus not only damages the offending agent, but incidentally produces damage to adjacent-tissue.