Immune complex hypersensitivity (Type III) reactions

Type III hypersensitivity occurs when antigen and antibody complexes accumulate when they are not adequately cleared by immune cells. They give rise to an inflammatory response and attract leukocytes. Type II immune complex hypersensitivity will occur when there is an excess of antigen leading to complexes that are not cleared from the circulation. It involves soluble antigens that are not bound to cell surfaces unlike those found in Type II hypersensitivity. When these antigens bind the antibodies, immune complexes of various sizes will form. Large complexes can be cleared by macrophages but they have difficulty in the disposal of small immune complexes. These immune complexes will insert themselves into smaller blood vessels and joints causing symptoms. Such depositions in tissues often stimulate an inflammatory response and they can cause damage wherever they precipitate.