Type I Hypersensitivity

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IMMUNOLOGY
HYPERSENSITIVITY


Introduction

  • Also known as IgE-mediated or anaphylactic hypersensitivity.
  • 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.
  • Affects face, eyes, nose and feet
  • IgE has a high affinity to IgE receptors on mast cells and basophils, and so binds these receptors.
  • The individual mechanisms of cell degranulation may only have a marginal effect, but the combination can be highly effective in removing parasite infection
  • Eosinophils can also target IgE bound to helminths.
    • May either be attracted to the site of inflammation or produced de novo in response to cytokines.
    • Binding triggers their degranulation.
      • Potent biological molecules are released.
        • Eosinophil peroxidase.
        • Eosinophil basic protein.
  • This system can be invoked with inappropriate and innocuous antigens.
    • E.g. pollen.
    • Under these circumstances we get hay fever.


Common allergens:

  • Proteins : Foreign serum
  • Plant pollens : Rye grass, ragweed, timothy grass, birch trees
  • Drugs : Penicillin, sulphonamides, local anaesthetics, salicylates
  • Foods : Nuts, seafood, eggs, milk
  • Insect products : Bee venom, wasp venom, dust mites faeces, flea saliva

Mechanism

IMAGE 1: Sensitisation phase1.jpg -Brian Catchpole RVC 2008
IMAGE 2: Immesdiate-type hypersensitivity-Brian Catchpole/M Maidment RVC 2008


1. Initial antigen exposure sensitises immune system (Image 1):

  • Allergen exposure causes IgE production.
  • IgE coat mast cells by binding to Fc receptors.


2. Subsequent exposure to the specific allergen (Image 2):

  • mast cell degranulation.
  • basophils with IgE receptors are recruited and also degranulate.


Mast cells secrete mediators which cause:

  • The release of inflammatary cytokines
    • Steroid mediators, e.g. prostaglandins and leukotriens
  • Chemoattractants (attracts mainly neutrophils but also eosinophils, monocytes and basophils and plasma)
  • Some mast cell cytokines (especially eosinophils) release factors which promote growth and maturation of leukocytes
  • Vasoactive and inflammatory peptides (e.g. histamine and serotonin) which causes acute contraction of smooth muscle fibres
    • Leading to bronchoconstriction and vasoconstriction
    • Violent constriction can lead to the expulsion of parasites
  • The release of mast cell proteases
    • Promotes mucous production
    • Inhibits parasite interactions with the epithelial surface
  • Vasodilation (leads to redness)
  • Oedema (from leaky blood vessels)


3. The late phase response:

  • Mediated by eosinophils.
  • Takes longer (several hours) as the eosinophils are mobilised from the bone marrow.


Examples of Type I hypersensitivity

  • Atopy
  • Flea allergic dermatitis
  • Sweet itch