Disorders in Immunity

Diagnosing Allergies

Diagnosis of allergies involves several different levels of testing that can include nonspecific, specific, in vitro, and in vivo methods.

Allergies can result in serious, life-threatening hypersensitivity reactions. Even less severe allergies can cause unpleasant symptoms that make day-to-day life more difficult. Therefore, it is important to diagnose the offending allergen causing the reaction in patients so that they can take appropriate measures to avoid it. Specific testing for allergens depends on the type of allergic reaction and the likelihood of various allergens. For example, if patients present with symptoms of hay fever (a pollen allergy), staining and viewing nasal discharges under a microscope can aid in diagnosis. The presence of mast cells, eosinophils, and basophils in the discharge is characteristic of hay fever and distinguishes it from other disorders with similar symptoms. Mast cells release histamine during allergic reactions. Eosinophils are white blood cells that increase in number in infected or inflamed tissue and contain granules that stain with the dye eosin. Basophils are a type of white blood cell that increases in number in infectious and inflammatory reactions.

Another common in vitro, or out of body, test includes blood sampling. Nonspecific blood tests involve quantifying the total amount of immunoglobulin E (IgE) circulating in the patient's blood. Specific blood tests challenge the blood against specific allergens. There are several types of specific blood tests, and all involve IgE in the blood sample reacting in an allergen-specific manner. Blood tests are safe, minimize patient discomfort, and are reasonably effective. Because of safety concerns, blood tests are generally preferred for suspected food allergies. The blood test can measure the presence of IgE antibodies for specific food types.

In vivo allergy diagnosis tests are also available. These tests involve challenging the patient against the allergen directly. The skin prick test is the most common of allergy tests. In a skin prick test, a drop of solution containing an allergen is placed on the skin, typically of the forearm, and a needle is inserted through the solution and into the skin. The area of the prick is monitored for the formation of redness, swelling, or a fluid-filled blister, indicating a positive test. These skin changes are referred to as a wheal-and-flare reaction. A similar test places standardized patches with allergens in them on a patient's skin. After incubation, these patch tests are evaluated for the formation of eczema. Eczema, also called dermatitis, is a disease characterized by skin swelling and typically involves an itchy, inflamed skin rash. Eczema that develops at the patch placement site is diagnostic for the allergy.
The skin prick test is used to determine what might be causing a person's allergic responses. In this test, samples of common and/or suspected allergens are placed on the individual's skin and a needle is used to prick a small hole in the same area, such that the allergen is able to penetrate the skin. This area of the skin is then monitored for any signs of an allergic response, such as redness and swelling.
Credit: CDC/Dr. Frank Perlman, M.A. Parsons