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| ==Introduction== | | ==Introduction== |
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| 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]]). |
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| 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. |
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| ==Classification== | | ==Classification== |
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| 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. |
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| 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. |
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| !width="200"|Immune mechanism | | !width="200"|Immune mechanism |
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− | | 2-30 min || [[Type I Hypersensitivity|I]] || [[IgE]] mediated | + | | 2-30 min || <center><big>'''[[Type I Hypersensitivity|I]]'''</big></center> || <center>[[IgE]] mediated</center> |
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− | | 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> |
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− | | 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> |
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− | | 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> |
| |} | | |} |
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