TYPE III: IMMUNE COMPLEX
These reactions result from the presence of either circulating immune complexes in the tissues. Deposition of immune complexes depends on their size, charge, local concentration of complement, and perhaps most important the nature of the antigen. An excellent example of this type of reaction is the arthritis reaction in which antigen is injected into the skin of an animal previously sensitized to the same antigen and has produced antibody to that antigen. The preformed antibody goes to the site of the injected antigen and forms a complex, thereby inducing complement activation and neutrophil attraction. The result is intense local inflammation, hemorrhage and necrosis, There are numerous examples of this type of hypersensitivity reaction, including serum sickness, glomerulonephritis and systemic lupus erythemathosus.