Immune System

Definition

Allergy tests may be of two general types. In vivo tests that measure the immune response to an agent called an allergen that induces an allergic (atopic) reaction, and in vitro tests that measure the antibodies that mediate an allergic response. Such antibodies are those of the immunoglobulin E class (IgE) which have epsilon heavy chains which attach to mast cells.

Purpose

Allergy tests are performed to determine the cause of a person's allergic reaction. An allergic reaction is caused by the production of specific IgE antibodies against one or more antigens. Those antigens that elicit IgE production are termed allergens and are usually harmless substances. Some common allergens are described below.

• house dust mites and their waste
• hair, dander, and saliva of animals with fur or feathers
• cockroaches and their waste
• weed, grass, and tree pollens
• mold and mildew spores
• stinging insects such as bees, wasps, yellow jackets, hornets, and fire ants
• drugs such as penicillin, and sulfa drugs
• foods such as eggs, milk, nuts, and seafood
• ingredients found in dyes, cosmetics, and latex

Precautions

While allergy tests are safe for most people, there is always a possibility that the subject will experience a severe allergic reaction to an allergen used for in vivo testing. For this reason, the subject should be exposed to the lowest dose of allergen in the first phase of testing. In vitro blood tests are particularly appropriate for persons who have a history of severe allergy. In vitro testing may also be more appropriate for persons taking antihistamines, neuroleptics, or antidepressants, and for those with skin conditions all of which may interfere with skin test results.

Description

An allergic reaction is caused when a person's immune system produces IgE antibodies in response to a foreign antigen (allergen). IgE molecules are tightly bound to the surface of mast cells (and basophils in blood). These cells contain granules that have a high concentration of histamine and other substances that are responsible for the allergic reaction. Upon subsequent exposure to the same antigen, an immediate (type 1) hypersensitivity reaction called the atopic or allergic reaction ensues. The allergen binds to the IgE and the crosslinking of antigen and antibody molecules causes the mast cell to degranulate. The histamine and other allergic mediators are released and cause local swelling (edema) and redness (vasodilation). These reactions occur immediately and may be sufficient in intensity to cause constriction of the bronchi and shock. Such a systemic response to an allergen is called an anaphylactic reaction. Allergens most often responsible for anaphylactic reactions are insect bites and penicillin in persons who are allergic to these agents.

Allergies may be seasonal or chronic depending upon the exposure to the allergen. An allergy may be influenced by factors such as emotional stress, fatigue, infection, air pollution, and weather changes that can cause day to day variation in the severity of symptoms. These triggering factors add to what doctors call the "allergic load," the amount of allergens the body can tolerate at any given time without the occurrence of symptoms.

Allergies can manifest themselves in several ways. Conditions commonly caused by allergies include rhinitis, asthma, contact dermatitis, allergic gastritis, urticaria (hives), and conjunctivitis. Allergy tests are used to identify the allergen(s) responsible for the allergy. Once the allergen is known, avoidance and desensitization (subcutaneous injection of allergen extract) can be added to the treatment modality.

Types of allergy tests

IN VIVO TESTING. Known as a skin or scratch test, this method is most commonly performed by pricking the skin at multiple sites. Each site is tested by scratching the skin surface with a sterile needle laden with an allergen extract or placing allergen extract on the skin and then pricking the skin with a needle.

The advantages of this testing method include speed (results are immediately available), cost-effectiveness, and a high degree of sensitivity. The disadvantages of this method include patient discomfort, a high incidence of false positives, interference from antihistamines, other drugs and skin conditions.

IN VITRO TESTING. Immunoassays that measure total and specific IgE antibodies are used for in vitro allergy testing. The most commonly used method for specific IgE testing is called the RAST test (radioallergosorbent test). This test is a variation of an indirect antiglobulin

test. The allergen is bound to a solid phase such as a plastic test tube or disc. Serum is added and incubated with the allergen. If IgE antibodies specific for the allergen are present, these bind to the tube or disc. The tube is washed, and anti-human IgE tagged with radioactive iodide is added. This attaches to the allergen-antibody complexes. After incubation, the tube is washed again, and its radioactivity is measured. The amount of radioactivity is directly proportional to the IgE concentration. This can be quantified by comparing the radioactivity of the test to standards. Standards consist of a specific IgE of known concentration (e.g., anti-birch) that is reacted with the respective purified allergen (birch extract). The RAST test can be prepared as a screening test by combining groups of allergens onto a single disc. Results are compared to a low-level IgE standard which serves as a positive cutpoint. Radioactivity above the standard indicates a positive test for allergy, and subsequent tests are performed using the individual allergens to determine the cause of a positive screen.

The immunoassay most commonly used for measuring total IgE is called the PRIST test (paper radioimmunosorbent test). This test is a double antibody sandwich type radioimmunoassay in which the IgE is the antigen. Total IgE in serum can also be determined by immunoassay methods. A positive test for total IgE indicates a diagnosis of allergy when allergic symptoms are present. However, serum IgE levels may be increased in persons with parasitic infections and malignant diseases in the absence of allergy.

In vitro tests have the advantage of being sensitive and specific although they are somewhat less sensitive than skin tests. The reagent quality is highly consistent from lot to lot. The methods are more reproducible and accuracy can be monitored with quality control. In vitro methods can be used for persons who are being treated with antihistamines and other medications. One venipuncture replaces multiple skin scratches that are painful and not well tolerated by young children. Disadvantages of in vitro tests are higher costs and longer turnaround time than skin tests(specific IgE tests require 1-2 days before results are available because of long incubation times).

OTHER TESTING. Provocation testing may be performed to positively identify suspected allergens after preliminary skin testing. A purified preparation of the allergen is inhaled or ingested in increasing concentrations to determine if it will provoke a response.

Inhalation testing is performed only after a patient's lung capacity and response to the medium used to dilute the allergen has been determined. Once this has been determined, the patient inhales increasingly concentrated samples of a particular allergen, followed each time by measurement of the exhalation capacity. Only one allergen is tested per day.

Provocation testing with food is more tedious than inhalation testing. First the suspect food is removed from the patient's diet for two weeks; then the patient eats a single portion of the suspect food and is monitored for a reaction.

A double-blind food challenge may also be used for diagnosis of food allergy. During this test, various foods, some of which are suspected of inducing an allergic reaction, are each placed in individual capsules; the patient swallows a capsule and is then watched to see if a reaction occurs. Persons with a history of severe allergic food reactions cannot be tested this way.

Preparation

Skin testing is preceded by a brief examination of the skin. The patient should refrain from using allergy medications for at least 48 hours before testing.

Immunoassay and RAST tests require a sample of blood, which is obtained via venipuncture by a nurse or phlebotomist followng standard precautions for the prevention of transmission of bloodborne pathogens.

Aftercare

Patients receiving skin tests should be monitored for 30 minutes following testing, and treated promptly should they develop signs of a severe allergic reaction. Occasionally, a delayed anaphylactic response may occur that will require immediate care; therefore, patient education regarding how to recognize delayed anaphylaxis is vital. The generalized redness and swelling that may occur in the skin test area will usually resolve within a day or two. More severe reactions may require topical or antihistamine therapy.

Inhalation tests may cause delayed asthma attacks, even if the antigen administered in the test initially produced no response. Severe initial reactions may justify close professional observation for at least 12 hours after testing.

Complications

Intradermal testing may inadvertently result in the injection of the allergen into the circulation, with an increased risk of adverse reactions. Inhalation tests may provoke an asthma attack. Exposure to new or unsuspected allergens in any test carries the risk of anaphylaxis.

Results

Skin testing

Lack of redness or swelling on a skin test indicates no allergic response. A wheal (an area of redness and swelling) exceeding 7 mm in diameter or larger than the histamine control, has a higher diagnostic value than smaller wheals.

Immunoassay testing

SPECIFIC IGE. Quantitative specific IgE cutoffs are the lowest radioactivity levels encountered in the highest dilution of standard used. The IgE level may be reported in RAST units, or as positive, equivocal, or negative.

TOTAL IGE. The upper limit of normal for total IgE is highly age dependent for children. The upper limit increases over the first 10 years, then levels off. The cutoff for adults varies with the test methodology. For the PRIST test the cutoff is approximately 25 kU/L IgE when the standard used is traceable to the 2nd International Reference Preparation of the World Health Organization.

Inhalation testing

In an inhalation test, the exhalation capacity should remain unchanged. Following allergen inhalation, reduction in exhalation capacity of more than 20%, and for at least 10-20 minutes, indicates a positive reaction to the allergen.

Food testing

In a food challenge, no symptoms should occur. Gastrointestinal symptoms within 24 hours following the ingestion of a suspected food allergen indicates a positive response.

Health care team roles

Allergy tests are ordered by a physician. Skin tests and inhalation tests are performed in a doctor's office by a nurse or healthcare worker specially trained to administer the test. A physician, nurse, or physician assistant should monitor patient for signs of anaphylaxis for 30 minutes after testing.

In vitro allergy tests are performed by clinical laboratory scientists CLS(NCA)/medical technologists MT(ASCP). Results are interpreted by a physician usually a specialist in immunologic and allergic diseases.

KEY TERMS

Allergen—A substance that provokes an allergic response.

Anaphylaxis—Increased sensitivity caused by previous exposure to an allergen that can result in blood vessel dilation (swelling) and smooth muscle contraction. Anaphylaxis can result in shock caused by a sharp fall in blood pressure.

Antibody—A specific protein produced by the immune system in response to a specific foreign protein or particle called an antigen.

Antigen—A foreign protein to which the body reacts by making antibodies.

Histamine—A chemical released by mast cells that causes vasodilation and bronchial constriction.

Mast cells—A tissue analog of the blood basophil, mast cells are prevalent near small blood vessels in the skin, membranes, and bone marrow. Mast cells bind a type of antibody called immunoglobulin E (IgE) on their surface, and participate in the allergic response by releasing histamine from intracellular granules.