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 (i.e. sound, touch, light, etc.) and/or emotional stimuli. Hypersensitivity reactions can be classified into four types. Type I: IgE mediated immediate reaction Type II: Antibody-mediated reaction (IgG or IgM antibodies) Type III: Immune complex-mediated reaction Type IV: Cytotoxic, 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. Gell and Coombs classification 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. Type I hypersensitivity Etiology Type I hypersensitivity occurs as a result of exposure to an antigen. The response to the antigen occurs in two stages: the sensitization and the effect stage. In the "sensitization" stage, the host experiences an asymptomatic contact with the antigen. Subsequently, in the "effect" period, the pre-sensitized host is re-introduced to the antigen, which then leads to a type I anaphylactic or atopic immune response. Types of antigens involved Food: nuts, eggs, soy, wheat, shellfish, etc. Animal source: bee, wasp, cats, insects, rats, etc. Environmental factors: dust mites, latex, pollen, mold, etc. Atopic diseases: allergic asthma, allergic rhinitis, conjunctivitis, dermatitis, etc. Medication-induced reactions: antibiotics Type II hypersensitivity Type II hypersensitivity reaction refers to an antibody-mediated immune reaction in which antibodies (IgG or IgM) are directed against cellular or extracellular matrix antigens with the resultant cellular destruction, functional loss, or damage to tissues. Damage can be accomplished via three different mechanisms: Antibody binding to cell surface receptors and altering its activity Activation of the complement pathway. Antibody-dependent cellular cytotoxicity. The pathophysiology of type II hypersensitivity reactions can be broadly classified into three types: Cell depletion or destruction without inflammation Inflammation mediated by complement or Fc receptor Cellular dysfunction by antibodies The process involves a series of immune-mediated events