Allergy laboratory

Skin prick test

Summary

Science

Clinical

Science

IgE, a special type of antibody, mediates Allergic (type 1 hypersensitivity) reactions. When it encounters a specific allergen IgE activates specialised cells called mast cells to produce chemicals such as histamine and leukotrienes that cause the visible symptoms typical of an allergic response. The skin prick assay is an ‘exposure’ test, that uses this hypersensitivity reaction, under controlled conditions, to identify what a person may be allergic to by measuring the overactive immune response to the potential allergen.

In the allergy clinic, the patient is asked to sit and role their sleeves up. A marker pen is used to identify on the arm where the allergens are to be applied. For every different allergen a solution is prepared that contains the proteins that may cause an adverse reaction. Some of these solutions are commercially produced specifically for skin prick testing but they can also be prepared in the clinic using fresh extract from the substance suspected of causing the allergy. A fine needle is used to gently prick the skin under the allergen solution. If IgE is present that specifically recognises the allergen it will activate mast cells to produce histamine, the outcome of this will be a raised circle of reddened skin called a wheal and flare around the point of allergen application. The presence of this wheal indicates that the patient has become sensitised to the particular substance via a previous exposure. To check if the test is working correctly a positive control is applied that contains histamine and should produce a reaction in all individuals. Also a negative control is used, this is simply a saline (salty) solution.

• Background
• Clinical