Diagnosis - Allergy and Hypersensitivity Disorders

Diagnosis

·        History: do symptoms occur in particular environments, particular times or seasons, what are dominant symptoms (eg sneezing/itching more likely to be allergy than chronic nasal blockage)

Skin prick tests

·        Useful for atopic allergies: especially of the mucous membranes

·        Use standard panel of allergen extracts (eg grass pollen, house dust mite, cat dander, etc)

·        Read wheal and flare reaction 10 – 15 minutes later

·        Safe, accurate and cheap 

·        If they have had an anaphylactic reaction ® test in a hospital setting. If very highly sensitive ® systemic reaction. If anaphylactic reaction, test tryptase (elevated for 1 – 6 hours) 

·        Skin prick tests will be negative in hypersensitivity that is not IgE mediated

·        Pathophysiology: 

o   Allergen + specific IgE (attached to mast cells) ® mast cell activation ® degranulation ® histamine and tryptase release + newly generated mediators (arachidonic acid metabolites eg leukotrienes and PGs) ® local & systemic effects. Eosinophils may also produce mediators of inflammation 

·        In the sensitisation process antigen presenting cells (dendritic cells, macrophages) present allergen fragments (epitopes) to T helper cells using MHC Class II. Mainly Th2 cells involved in inducing allergic disease ® IL4, IL5, IL3 ® IgE production, eosinophil growth and differentiation and mast cell growth

Lab tests

·        Tests for IgE antibodies (eg RAST tests) are available for certain allergens.  But also rises for parasite –       where in the world were they raised – this could be a cause

·        Indicate immune sensitisation only.  Allergy requires symptoms following exposure

·        Expensive and less accurate than skin tests, but useful if skin tests not possible, history of anaphylaxis

Challenge tests

·        Mainly in research setting

·        Food allergy: can do double blind, placebo-controlled food challenge. In small kids, removing food allergens from diet will improve severe eczema – but not in adults

·        Inhalation of cold air, histamine or exercise may be useful in assessing bronchial hyperreactivity in asthma

Contact Allergy

·        Affect whole skin and are usually life long

·        Diagnosed using patch testing on back for 48 hours. Difficult to distinguish between allergic and irritant reactions