Antigen bound to antibody. Diseases resulting from immune complex formation can be placed broadly into three groups :

• The combined effects of a low-grade persistent infection (such as occur with haemolytic Streptococcus viridans or staphylococcal infective endocarditis, or with a parasite such as Plasmodium vivax, or in viral hepatitis), together with a weak antibody response, leads to chronic immune complex formation with the eventual deposition of complexes in body tissues.

• Immune complex disease is a frequent complication of autoimmune disease where the continued production of antibodies to a self-antigen leads to prolonged immune complex formation. The mononuclear phagocytes, erythrocytes, and complement systems (which are responsible for the removal of complexes) become overloaded and the complexes are deposited in body tissues, as occurs in systemic lupus erythematosus .

• Immune complexes may be formed at body surfaces, notably in the lungs following repeated inhalation of antigenic material from moulds, plants or animals. This is exemplified in Farmer's lung and Pigeon fancier's lung, where there are circulating antibodies to the actinomycete fungi found in mouldy hay, or to pigeon antigens. Both diseases are forms of extrinsic allergic alveolitis and they only occur after repeated exposure to the antigen.