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35.2 Mechanism and classification of hypersensitivity reactions

Based on immune response and some miscellaneous factors, hypersensitivity reactions are classified as follows

Type I hypersensitivity or immediate hypersensitivity

It is characterized by the stimulation of helper T cells that are associated with production of IgE antibodies and inflammation. Type I is the most common hypersensitive reaction. Atopy or allergic reaction is the best example of type I reactions.

Type II hypersensitive disorders

This occurs due to activation of complement system by IgG and IgM antibodies. Some of these antibodies are specific for some antigens and the disease caused by such antibodies are called type II hypersensitive disorders such as Graves' disease.

Type III hypersensitive disorders

Various other antibodies make immune complexes in blood circulation and cause tissue damage. Such immune complex diseases are called type III hypersensitive disorders. Arthus reaction is a type III hypersensitive disorder.

Type IV hypersensitive disorder

It involves activation of phagocytes, T-lymphocytes, and natural killer cells. Multiple sclerosis is one of such kind.

In brief, majority of hypersensitive reactions are caused by stimulation of subset of T helper cells. They generally induce inflammation and tissue damage by recruiting neutrophils and macrophages.