A hypersensitivity reaction is an exaggerated or inappropriate immune response to an antigen or allergen. There are four traditional classifications for hypersensitivity reactions, which include:
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Type I Hypersensitivity: Also known as immediate hypersensitivity, this reaction involves immunoglobulin E (IgE)-mediated release of antibodies against the soluble antigen. This results in mast cell degranulation and release of histamine and other inflammatory mediators. Examples include anaphylaxis and allergic rhinoconjunctivitis.
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Type II Hypersensitivity: Also known as cytotoxic reactions, this reaction engages IgG and IgM antibodies, leading to complement system activation and cell damage or lysis. An example is drug-induced hemolytic anemia.
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Type III Hypersensitivity: Also known as immune complex reactions, this reaction involves IgG, IgM, and sometimes IgA antibodies. The build-up of these immune complexes results in complement system activation, which leads to polymorphonuclear leukocytes (PMNs) chemotaxis and eventually causes tissue damage. An example is serum sickness.
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Type IV Hypersensitivity: Also known as delayed-type, this reaction involves T-cell-mediated reactions. T-cells or macrophages are activated as a result of cytokine release, leading to tissue damage. An example is contact dermatitis from poison ivy or nickel allergy.
Symptoms of hypersensitivity reactions depend on the type of reaction and can range from mild to life-threatening. Some of the symptoms or conditions caused by hypersensitivity reactions include hay fever, eczema, hives, anaphylaxis, and autoimmune diseases.
