33. Differential diagnosis of allergy and atopia.

Page created on February 21, 2022. Not updated since.


Atopy is a personal or familial tendency to develop IgE-mediated hypersensitivity reactions, predisposing to asthma, eczema (atopic dermatitis), hay fever (allergic rhinitis and conjunctivitis), and food allergies, as well as less common disorders like eosinophilic oesophagitis.

Allergy is another name for a hypersensitivity reaction, either IgE-mediated or non-IgE-mediated. Severe allergy can cause anaphylaxis, a rapid-onset, potentially life-threatening reaction with angioedema, bronchospasm, and possibly distributive shock.

Most allergies occur in response to one of these allergens:

  • Plant pollen
  • House-dust mite
  • Pet dander
  • Foods (egg, milk, nuts, wheat)
  • Insect bites/stings
  • Drugs
  • Latex


People with allergic disorders usually have more than one, so it’s important to ask about and examine the eyes, nose, skin, and lungs. Atopic dermatitis usually affects the limb flexures. A child breathing with their mouth likely has an obstructed nasal airway. It’s also important to ask about when the symptoms occur.

Skin prick testing can be used to diagnose allergy. A drop of allergen is placed on the skin, after which the skin is pricked (which introduces the allergen beneath the skin barrier). If a wheal (urtica) develops, the test is positive for that allergen.

Nowadays there are also allergen-specific blood tests which can give information as to which allergen the person is allergic to. An elevated total IgE is evidence of atopy. In case of doubtful cases, a food challenge can be tried, but only in a situation in which anaphylaxis can be rapidly treated.

Most common allergic disorders

Disorder Typical features
Allergic rhinitis and conjunctivitis (hay fever) Runny nose, conjunctivitis, itchy nose, itchy eyes, itchy throat, sneezing, nasal obstruction
Allergic asthma Wheezing, dyspnoea, dry cough
Atopic dermatitis Dry, itchy skin, usually in limb flexures
Food allergy Either acute reaction (IgE-mediated) with widespread urticaria, facial swelling, etc., or non-acute reaction (non-IgE mediated) with diarrhoea, failure to thrive, etc.
Urticaria Hives, erythema, itchiness

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