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==Introduction==
==Introduction==
* Also known as [[Immunoglobulin E - WikiBlood|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.
* Generally affects face, eyes, nose and feet
* Generally affects face, eyes, nose and feet
* [[Immunoglobulin E - WikiBlood|IgE]] has a high affinity to IgE receptors on [[Mast Cells - WikiBlood|mast cells]] and [[Basophils|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 which elict a type I hypersensitivity reaction:==
[[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]]
'''2. Subsequent exposure to the specific allergen (Image 2):'''
'''2. Subsequent exposure to the specific allergen (Image 2):'''
* [[Mast Cells - WikiBlood|Mast cells]] degranulate.
* [[Mast Cells|Mast cells]] degranulate.
* [[Basophils|Basophils]] with [[Immunoglobulin E - WikiBlood|IgE]] receptors are recruited and also degranulate.
* [[Basophils|Basophils]] with [[Immunoglobulin E|IgE]] receptors are recruited and also degranulate.
Mast cells secrete mediators:
Mast cells secrete mediators:
* The release of inflammatary [[Cytokines - WikiBlood|cytokines]] which can cause the dog to become itchy.
* 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 - WikiBlood|eosinophils]], [[Monocytes|monocytes]] and [[Basophils|basophils]] and plasma)
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
2. Vasoactive and inflammatory peptides (e.g. histamine and serotonin) which causes acute contraction of smooth muscle fibres
'''3. The late phase response:'''
'''3. The late phase response:'''
* Mediated by [[Eosinophils - WikiBlood|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===
* [[Allergic diseases - WikiClinical#Allergic diseases#1. Atopic dermatitis - Dogs and horses|Atopy]]
* [[Atopic Dermatitis|Atopy]]
* [[Allergic diseases - WikiClinical#Allergic diseases#2. Flea allergic dermatitis (FAD)- Dogs and cats|Flea allergic dermatitis]]
* [[Flea Allergic Dermatitis|Flea allergic dermatitis]]
* [[Allergic diseases - WikiClinical#Allergic diseases#3. Culicoides hypersensitivity (Sweet itch) - Horses|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]]