Hypersensitivity (also called hypersensitivity reaction or intolerance) refers to undesirable reactions produced by the normal immune system, including allergies and autoimmunity. They are usually referred to as an over-reaction of the immune system and these reactions may be damaging and uncomfortable. This is an immunologic term and is not to be confused with the psychiatric term of being hypersensitive which implies to an individual who may be overly sensitive to physical (ie sound, touch, light, etc.) and/or emotional stimuli. Although there is a relation between the two – studies have shown that those individuals that have ADHD (a psychiatric disorder) are more likely to have hypersensitivity reactions such as allergies, asthma, eczema than those who do not have ADHD.
Hypersensitivity reactions can be classified into four types.
Type I: IgE mediated immediate reaction
Type II: Antibody-mediated cytotoxic reaction (IgG or IgM antibodies)
Type III: Immune complex-mediated reaction
Type IV: Cell-mediated, delayed hypersensitivity reaction
The first three types are considered immediate hypersensitivity reactions because they occur within 24 hours. The fourth type is considered a delayed hypersensitivity reaction because it usually occurs more than 12 hours after exposure to the allergen, with a maximal reaction time between 48 and 72 hours.
The Gell and Coombs classification of hypersensitivity is the most widely used, and distinguishes four types of immune response which result in bystander tissue damage.