Type I Hypersensitivity

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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