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==Introduction==
==Introduction==
* Also known as IgE-mediated or anaphylactic hypersensitivity.
* Also known as [[Immunoglobulin E|IgE]]-mediated or anaphylactic hypersensitivity.
* Ranges from mild cases, such as hayfever, to life-threatening reactions, such as bee-stings.
* Ranges from mild cases, such as hayfever, to life-threatening reactions, such as bee-stings.
* Antigens that induce a type I reaction are known as allergens.
* Antigens that induce a type I reaction are known as allergens.
* Affects face, eyes, nose and feet
* Generally affects face, eyes, nose and feet
* IgE has a high affinity to IgE receptors on mast cells and basophils, and so binds these receptors.
* [[Immunoglobulin E|IgE]] has a high affinity to IgE receptors on [[Mast Cells|mast cells]] and [[Basophils|basophils]], and so binds these receptors.
==Common allergens which elict a type I hypersensitivity reaction:==
==Common allergens:==
* Proteins : Foreign serum
* Proteins : Foreign serum
[[Image:Sensitisation phase1.jpg|right|thumb|150px|IMAGE 1: Sensitisation phase1.jpg
[[Image:Sensitisation phase1.jpg|right|thumb|150px|IMAGE 1: Sensitisation phase1.jpg
-Brian Catchpole RVC 2008]]
-Brian Catchpole RVC 2008]]
[[Image:Immediate-type hypersensitivity.jpg|right|thumb|150px|IMAGE 2: Immesdiate-type hypersensitivity-Brian Catchpole/M Maidment RVC 2008]]
[[Image:Immediate-type hypersensitivity.jpg|right|thumb|150px|IMAGE 2: Immediate-type hypersensitivity-Brian Catchpole/M Maidment RVC 2008]]
* Allergen exposure causes IgE production.
* Allergen exposure causes IgE production.
* IgE coat mast cells by binding to Fc receptors.
* IgE coat mast cells by binding to Fc receptors.
* Mast cells are now sensitised to this particular allergen.
'''2. Subsequent exposure to the specific allergen (Image 2):'''
'''2. Subsequent exposure to the specific allergen (Image 2):'''
* mast cell degranulation.
* [[Mast Cells|Mast cells]] degranulate.
* basophils with IgE receptors are recruited and also degranulate.
* [[Basophils|Basophils]] with [[Immunoglobulin E|IgE]] receptors are recruited and also degranulate.
Mast cells secrete mediators which cause:
Mast cells secrete mediators:
* The release of inflammatary cytokines
* The release of inflammatary [[Cytokines|cytokines]] which can cause the dog to become itchy.
**Steroid mediators, e.g. prostaglandins and leukotriens
**Steroid mediators, e.g. prostaglandins and leukotriens
1. Chemoattractants (attracts mainly [[Neutrophils|neutrophils]] but also [[Eosinophils|eosinophils]], [[Monocytes|monocytes]] and [[Basophils|basophils]] and plasma)
2. Vasoactive and inflammatory peptides (e.g. histamine and serotonin) which causes acute contraction of smooth muscle fibres
**Leading to bronchoconstriction and vasoconstriction
*If the allergen is inhaled (locally) it can lead to bronchoconstriction.
3. Mucus production - due to the release of mast cell proteases
4. Vasodilation (leads to redness and heat).
5. Oedema (from leaky blood vessels).
'''3. The late phase response:'''
'''3. The late phase response:'''
* Mediated by eosinophils.
* Mediated by [[Eosinophils|eosinophils]].
* Takes longer (several hours) as the eosinophils are mobilised from the bone marrow.
* Takes longer (several hours) as the eosinophils are mobilised from the bone marrow.
===Examples of Type I hypersensitivity===
===Examples of Type I hypersensitivity===
* Atopy
* [[Atopic Dermatitis|Atopy]]
* Flea allergic dermatitis
* [[Flea Allergic Dermatitis|Flea allergic dermatitis]]
* Sweet itch
* [[Culicoides Hypersensitivity|Sweet itch]]
==From Pathology==
*Mediated by pharmacologically active substances from [[Mast Cells|mast cells]] and [[Basophils|basophils]]
**Due to antigen-antibody (usually [[IgE]]) binding to receptors on those cells
*Substances include histamine, serotonin, leukotriens, prostaglandins
*Can be systemic or local
*Skin becomes pruritic, raised erythematous borders of wheals
*Immediate reaction
*Includes:
**[[Atopy|Atopic dermatitis]]
**[[Urticaria]]
**Angioedema
**Fly bite hypersensitivity
**Gastrointestinal parasites
**Food
*Microscopically:
**Capillary dilation, oedema, mast cell degranulation, eosinophil infiltration
{{review}}
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{{Jim Bee 2007}}
[[Category:Hypersensitivity]]