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Hypersensitivity - Introduction (view source)
Revision as of 17:12, 17 March 2012
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==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. [[:Category:Allergic Diseases|allergy]]) or directed at self antigens (ie. [[Autoimmune Diseases - Introduction|autoimmunity]]).
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. [[:Category:Allergic Diseases|allergy]]) or directed at self antigens (ie. [[Autoimmune Diseases - Introduction|autoimmunity]]).
These reactions are antigen specific, occurring only after the immune system has already been sensitised to that antigen. This indicates the [[:Category:Adaptive Immune System|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|(type IV hypersensitivity)]] is not seen for days and is generally cell-mediated.
These reactions are antigen specific, occurring only after the immune system has already been sensitised to that antigen. This indicates the [[:Category:Adaptive Immune System|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|(type IV hypersensitivity)]] is not seen for days and is generally cell-mediated.
==Classification==
==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.
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.
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.
Types I, II and III are antibody-mediated; type IV is cell mediated.
!width="200"|Immune mechanism
!width="200"|Immune mechanism
|-
|-
| 2-30 min || [[Type I Hypersensitivity|I]] || [[IgE]] mediated
| 2-30 min || <center><big>'''[[Type I Hypersensitivity|I]]'''</big></center> || <center>[[IgE]] mediated</center>
|-
|-
| 5-8 h || [[Type II Hypersensitivity|II]] || [[IgG]] mediated: fixed antigen
| 5-8 h || <center><big>'''[[Type II Hypersensitivity|II]]'''</big></center> || <center>[[IgG]] mediated: fixed antigen</center>
|-
|-
| 2-8 h || [[Type III Hypersensitivity|III]] || [[IgG]] mediated: soluble antigen
| 2-8 h || <center><big>'''[[Type III Hypersensitivity|III]]'''</big></center> || <center>[[IgG]] mediated: soluble antigen</center>
|-
|-
| 24-72 h || [[Type IV Hypersensitivity|IV]] || T cell mediated
| 24-72 h || <center><big>'''[[Type IV Hypersensitivity|IV]]'''</big></center> || <center>[[T cells|T cell]] mediated</center>
|}
|}
* D.C. Blood and V.P. Studdert'''Saunders Comprehensive Veterinary Dictionary''' Elsevier Science 1999 2nd Edition
* D.C. Blood and V.P. Studdert'''Saunders Comprehensive Veterinary Dictionary''' Elsevier Science 1999 2nd Edition
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{{Jim Bee 2007}}
[[Category:Hypersensitivity]]
[[Category:Hypersensitivity]]