Allergy

Original article by Nicole McGrath | Last updated on 28/6/2014
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Definitions

Allergy – Inappropriate immune response to normally harmless allergen.
Allergen – Foreign particles which produce an IgE-mediated hypersensitivity reaction in sensitive individuals.
Atopygenetic predisposition to exaggerated immune responses

Pathology

  • Individuals are said to become ‘sensitized’ after initial exposure to antigen (allowing the development of antigen-specific IgE antibodies), and subsequent exposure will result in an allergic reaction.
  • Antigens can enter the body via the skin or the respiratory or gastrointestinal mucosa.
  • Allergens which enter the body bind to antigen-presenting cells which allow exposure to antigen-specific Th2 cells. These cells produce cytokines which activate mast cells which subsequently release histamine, leukotrienes and prostaglandins.
  • Histamine is responsible for local vessel dilation and increased permeability.
  • Activated Th2 cells also stimulate the production of allergen-specific IgE from B-cells. These antibodies are able to stimulate mast cell degranulation directly once bound to antigen.
  • Prolonged or repeated allergen exposure causes eosinophil activation which enhances local inflammation.

 

 

Epidemiology

  • 30% of the population develop an allergic disorder.
  • The most common allergens include house dust; dust mite faeces; animal dander; pollen (grass or tree); moulds and certain foods. 
  • Allergy is more likely if the exposure occurs early, and is likely to improve with age.
  • There is also a genetic component: if one parent affected the risk is 25-40%, if both parents are affected, the risk increases to 50-75%.
 

Signs and symptoms

These depend on the site of exposure, dose and individual.
Symptoms may include:
o   Urticarial or eczematous rash,
o   Asthma,
o   Rhinitis,
o   Conjunctivitis,
o   Diarrhoea and vomiting,
o   Anaphylaxis.
 

Diagnosis

  • A careful history may help identify trigger allergens and provide information about attack frequency and changes over time.
  • Family history.
  • Skin prick test - exposure to standardised allergen solution through forearm skin prick. A wheal >2mm larger than the negative (saline) control is a positive result.  
 

Treatment

  • Avoidance of allergen if possible.
  • Topical treatments: sodium cromoglicate (nasal spray/eye-drops) – stabilises mast cells to prevent degranulation; topical steroids – reduce vessel permeability and cytokine synthesis; emollient cream – reduce itching and water loss through damaged skin; bronchodilators.
  • Oral antihisatamines or steroids.
  • De-sensitisation therapy can be used in upper airway allergies if symptoms are not controlled on maximal medical therapy.