Hypersensitivity - Introduction
Introduction
Hypersensitivity is a hyperactive immune response to an antigen which leads to cell and tissue damage. It involves normal immune mechanisms directed towards harmless environmental antigens (ie. allergy) or directed at self antigens (ie. autoimmunity).
These reactions are antigen specific, occurring only after the immune system has already been sensitised to that antigen. This indicates the adaptive immune system is involved, and such responses can be either humoral or cell mediated. Those initiated by antibodies or antibody-antigen complexes are called immediate hypersensitivity, as the symptoms are seen within hours of exposure. Conversely, the response for delayed-type hypersensitivity (type IV hypersensitivity) is not seen for days and is generally cell-mediated.
It is important to note that these responses are part of normal immune defences- in the case of hypersensitivity however the keywords are exaggerated and damaging.
Classification
Hypersensitivity reactions can be classified according to the type of immune mechanism involved, based on a scheme proposed by Gell and Coombs over 40 years ago. There are four main types, although a fifth has recently been added.
Types I, II and III are antibody-mediated; type IV is cell mediated.
| Time of appearance | Type | Immune mechanism |
|---|---|---|
| 2-30 min | I | IgE mediated |
| 5-8 h | II | IgG mediated: fixed antigen |
| 2-8 h | III | IgG mediated: soluble antigen |
| 24-72 h | IV | T cell mediated |
References
Books
- Gareth Kitchen Immunology and Haematology Mosby Elsevier 2007 3rd Edition
- D.C. Blood and V.P. StuddertSaunders Comprehensive Veterinary Dictionary Elsevier Science 1999 2nd Edition