Type III hypersensitivity is caused by circulating immunocomplexes (see Fig. 2-29C) and is typified by serum sickness (a drug reaction in which multimeric drug-antibody aggregates form in solution). Preformed immunocomplexes deposit in various vascular beds and cause injury at these sites.
In type III hypersensitivity reaction, an abnormal immune response is mediated by the formation of antigen-antibody aggregates called "immune complexes." They can precipitate in various tissues such as skin, joints, vessels, or glomeruli, and trigger the classical complement pathway.
Examples of type III hypersensitivity reactions include drug-induced serum sickness, farmer's lung and systemic lupus erythematosus.
Type III reactions result when a person who has been strongly sensitized to a particular antigen is subsequently exposed to that antigen. In a type III reaction, the antigen-antibody complex becomes deposited on the walls of the small blood vessels.
What is the mechanism of type III hypersensitivity? Antibodies react to soluble antigen by forming lattices of antibody and antigen called an immune complex. When immune complexes (IgG or IgM) which are normally removed by macropahges in sleen or liver form in large amounts they overwhelm these coping strategies.
A good example of a type III Hypersensitivity is the autoimmune disease systemic lupus erythematosus, also just called lupus.