Type I Hypersensitivity

Type I Hypersensitivity


Introduction

  • Rapidly developing immunological reaction
  • Occur within minutes
  • Wheal & flare reaction
  • Combination of an antigen with antibody bound to mast cells in individuals previously sensitized to the antigen.
  • Mast cells are the most important cells.
  • Eosinophils are the most important cells in the late phase reaction .
  • Histamine is the most important mediator.
  • IL-4 is essential for turning on IgE secreting B cells.
  • PAF is the most important cytokine in the initiation of late phase reaction.
  • Most potent eosinophilic activating cytokine is IL-5. 

Exposure occurs

  • By inhalation (respiratory route)
  • Ingestion (GIT) of antigen.

Etiology

  • Genetic
  • Pollutants
  • Viral

Body parts generally they include:

  • nose: allergic rhinitis (hay fever)
  • eye: allergic conjunctivitis
  • skin: hives
  • soft tissue: angioedema
  • lungs and airways: asthma
  • generalised in the body: anaphylaxis and death if not treated.

Peak action time

  • 15-30 mins

Two phases :-

Initial response

  • After first antigen exposure, antigen presented to CD-4 helper T cells (TH2 type) by antigen presenting cells.
  • Primed TH2 cells release IL-4 that acts on B-cells to form Ig E specific for that particular antigen.
  • IgE bind to the surface receptors of mast cells and basophils.
  • This act as sensitization (prior sensitization)
  • First exposure is also called priming or sensitizing exposure (dose).
  • Subsequent exposure (shocking dose) to same antigen
  • Result in activation of mast cells and basophils with release of mediator from these cells that leads to:
  • Smooth muscle spasm
  • Increase mucus secretion from the epithelial cells.
  • Vasodilatation
  • Increased vascular permeability
  • Recuritment of inflammatory cells

Late phase response

  • Additional leukocytes are recruited.
  • Cells amplify and sustain the inflammatory response without additional exposure to the triggred antigen.
  • Eosinophils produce major basic protein and eosinophilic cataionic protein that are toxic to epithelial cells.
  • Platelet activating factor (PAF) recruits and activates inflammatory cells 
  • PAF important cytokine in the initatiation of late phase response.

Type I hypersensitivity occurs in two forms

  1. Anaphylaxis      →            Acute, potentially fatal, systemic.
  2. Atopy                 →           Chronic, Nonfatal, Localized.

Examples

  • Urticaria
  • angioedem
  • hay fever
  • some forms of asthma
  • eczema
  • Anaphylactic shock
  • Casoni’s test
  • Theobald Smith phenomenon
  • Schultz Dale phenomenon
  • Prausnitz Kustner (PK) reaction

 Type I hypersensitivity test

  • Injection is given subcutaneously or intradermally (usually in the forearm).
  • The site is examined after 20 minutes.
  • Examples → Casoni’s testsensitivity testing of drug.
Exam Question
 

Introduction

  • Rapidly developing immunological reaction
  • Occur within minutes
  • Wheal & flare reaction
  • Combination of an antigen with antibody bound to mast cells in individuals previously sensitized to the antigen.
  • Mast cells are the most important cells.
  • Eosinophils are the most important cells in the late phase reaction .
  • Histamine is the most important mediator.
  • IL-4 is essential for turning on IgE secreting B cells.
  • PAF is the most important cytokine in the initiation of late phase reaction.
  • Most potent eosinophilic activating cytokine is IL-5.

Examples

  • Urticaria
  • angioedem
  • hay fever
  • some forms of asthma
  • eczema
  • Anaphylactic shock
  • Casoni’s test
  • Theobald Smith phenomenon
  • Schultz Dale phenomenon
  • Prausnitz Kustner (PK) reaction

 Type I hypersensitivity test

  • Injection is given subcutaneously or intradermally (usually in the forearm).
  • The site is examined after 20 minutes.
  • Examples → Casoni’s test, sensitivity testing of drug.
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