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Hypersensitivity Type I

Hypersensitivity Type I
"Immediate hypersensitivity reactions"
IgE mediated diseases
Mechanism of tissue injury:
Allergen stimulates Th2.



Th2 secrets: “IL-4” which stimulate B cell antibody class switching to produce IgE that binds to mast cell surface.



The Fc portion of IgE binds to the surface of mast cells.
When the allergen cross-links the Fab portions of the mast cell-bound IgE, this triggers degranulation.


Cross linkage of IgE on the surface of Mast cell:

يعني ببساطة:
القصة بتبدأ من ال Th2
هيا اللي بتفرز “IL-4”  المسئول عن ال class switching of plasma cell to produce IgE insted of IgM
المشكلة ان ال IgE له مستقبلات علي سطح بعض الخلايا في الجسم زي  ال Mast cells & Basophiles
في حالة ارتباط ال IgE بسطح هذه الخلايا تصبح حساسه "قابلة للانفجار"  ....... sensitized
في حالة التعرض لل Allergen مرة اخري يرتبط بال IgE علي سطح الخلايا وهنا يحدث الانفجااااااار
Degranulation
See this movie:






1. Early phase:
Appears within minutes after exposure to the antigen.
Due to release of inflammatory mediators such as:

  • Histamine.


  • Platelet-activating factor.


  • Leukotrienes.


  • Bradykinins.


  • Prostaglandins.


  • Cytokines.

These inflammatory agents cause the following:
A. Vasodilatation:

  • This causes local redness (Erythema) at the site of allergen delivery.
  • If it is widespread, it can lead to a drop in blood pressure, and shock.
B. Increased capillary permeability:

  • This causes swelling of local tissues (Edema).
  • If widespread, it can lead to decreased blood volume and shock.
  • In Respiratory system, it causes broncho-constriction.
  • This leads to wheezing and difficulty in breathing (Asthma).
C. Stimulation of mucus secretion:

  •  This leads to narrowing of airways (Asthma).

D. Stimulation of nerve endings:


  • This leads to itching and pain in the skin.



2. Late phase:
May begin several hours after exposure to antigen.
Due to release of Cytokines such as:

  • TNF.


  • IL-4.

These Cytokines stimulate the recruitment of leukocytes.
The principal leukocytes involved in this reaction are:

  • Eosinophils.


  • Neutrophils.

These cells mediate the contanous inflamatory reaction, which leads to destruction.

Clinical syndromes:
Two major forms are commonly encountered:

Localized & Systemic reactions
Localized reaction: "Atopy"
  • Hay fever.
  • Bronchial asthma.
  • Food allergies.
  • Dermatitis.
Systemic reaction: "Anaphylactic shock"




 



Diagnosis:
1. Skin test: "Prick & intradermal test"
Scratch test commonly used where antigens are topically applied on skin and skin scratches are made through them.
The tests resulting in wheal and flare.

2- Assay of Serum IgE:
Total IgE & Specific IgE.
Measured by enzyme immunoassay (ELISA).
IgE may be elevated in some non-atopic diseases (e.g., myelomas, helminthic infection).

 




Pictures showing Scratch test which used for diagnosis Hypersensitivity type I
Also it helps in treatment
 

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