Type III (Immune Complex) Hypersensitivity- Mechanism, Examples


Hypersensitivity is a term that describes an abnormal or excessive immune response to an antigen, resulting in tissue damage or disease. There are four types of hypersensitivity reactions, classified by Gell and Coombs according to the mechanism and the type of antigen involved.

Type III hypersensitivity, also known as immune complex hypersensitivity, occurs when antibodies bind to soluble antigens in the circulation and form immune complexes that are not cleared by the innate immune system . These immune complexes can deposit in various tissues and organs, such as the skin, kidneys, joints, lungs, and blood vessels, and trigger an inflammatory response that causes tissue damage .

Type III hypersensitivity can be divided into two forms: local and systemic. Local immune complex disease, or Arthus reaction, occurs when immune complexes are formed locally in the tissues, usually as a result of repeated exposure to an antigen by injection or inhalation . Systemic immune complex disease occurs when immune complexes are formed in the blood and circulate throughout the body, depositing in different organs and causing widespread inflammation . Some examples of systemic immune complex diseases are systemic lupus erythematosus (SLE), post-streptococcal glomerulonephritis, serum sickness, and farmer`s lung .

Type III hypersensitivity is mediated by IgG and IgM antibodies that activate the classical pathway of complement . The complement activation produces anaphylatoxins (C3a and C5a) that attract neutrophils and monocytes to the site of immune complex deposition . These cells attempt to phagocytose the immune complexes but fail to do so because they are bound to the tissue . Instead, they release inflammatory mediators such as lysosomal enzymes, prostaglandins, and reactive oxygen species that damage the surrounding tissue . In addition, the Fc region of the antibody in the immune complex can bind to Fc receptors on platelets and cause aggregation and thrombosis .

Type III hypersensitivity is a serious condition that can lead to chronic inflammation, tissue necrosis, organ failure, and death if not treated promptly. The diagnosis of type III hypersensitivity is based on clinical features, laboratory tests (such as serum complement levels, anti-nuclear antibodies, anti-streptolysin O titers), and histopathology (such as vasculitis, glomerulonephritis, granulomas) . The treatment of type III hypersensitivity depends on the underlying cause and the severity of the symptoms. It may include anti-inflammatory drugs (such as corticosteroids), immunosuppressive drugs (such as cyclophosphamide), plasmapheresis (to remove circulating immune complexes), and supportive care (such as dialysis) .

In this article, we will explain the mechanism of type III hypersensitivity in more detail and describe some examples of local and systemic immune complex diseases. We will also discuss how type III hypersensitivity can be prevented and treated.