A delayed hypersensitivity test (DHT) is an immune function test measuring the presence of activated T cells that recognize a certain substance.
Purpose
The immune system protects against infection by viruses, bacteria, fungi, and parasites. After initial exposure to a foreign substance, or antigen, the immune system creates both antibodies and sensitized T cells. Both these immune agents respond when the body is reexposed to the antigen. Antibodies, which are circulating proteins, respond within minutes, to give what is termed an immediate hypersensitivity reaction. T cells' responses occur over several days, and are thus called delayed hypersensitivity reactions. The cascade of events initiated by the T cells leads to hardening (induration) and redness (erythema) at the injection site.
A DHT is performed for one of three reasons:
To test for exposure to specific diseases, such as tuberculosis (TB). Tuberculosis testing is done by injecting into the skin a small volume of TB antigen, which contains no organisms (live or dead) but can still provoke an immune response.
To test for allergic sensitivity to potential skin irritants, such as poison ivy. Skin allergy testing is usually done by placing a series of adhesive patches on the skin containing potential allergens, or allergy-causing substances.
To assess the vitality of the T cell response as part of the evaluation of immune system health in infection, cancer, immune disorders, pre-transplantation screening, aging, and malnutrition. DHT can help predict survival in immunocompromised patients, and evaluate the success of restorative therapy. Antigens used for these tests must be ones the patient has been exposed to before, and, therefore, include inactivated antigens from common infectious agents to which the patient might have been exposed, such as mumps, Candida albicans, tetanus toxoid, and trichophyton (a skin fungus).
Precautions
No special precautions are necessary for most patients. Those with known hypersensitivity to certain skin irritants should alert the clinician performing the test. Some commercial preparations of fungal antigens contain mercury, a source of irritation to some patients.
Description
The most accurate TB test is the Mantoux test, in which a small amount of TB antigen is injected into the skin. The area is examined 48–72 hours after the injection.
In the patch test, 20–30 adhesive patches are usually placed on the upper back. The patches are kept in place and the area is kept dry for 48 hours. The patches are then removed, and the skin is examined 24 hours afterward, and possibly again a day or more following that. Patch testing is usually performed following a patient complaint of skin irritation from an unknown substance. Testing may suggest several candidates; identifying the right one requires careful review of the patient's possible exposure.
The test of overall T cell responsiveness is performed with several injections. Each area injected is circled and marked. Results are read 48 hours after the injection.
Preparation
No special preparation is necessary.
Aftercare
Patches should be kept dry. Injection sites may be washed, but excessive rubbing should be avoided. Patches and injection sites may become reddened or irritated. If a patch causes severe itching or discomfort, the patient should remove it immediately.
Risks
DHT is quite safe for virtually all people. There is no risk of infection from the agents injected, since they are purified antigens, not whole organisms. Life threatening, hypersensitive reactions (anaphylaxis) are a very small risk; patients should notify the administering physician immediately if signs of wheezing, swelling, or diffuse redness of the skin develops.
Normal results
Absence of exposure to TB is indicated by absent or very little skin reaction; redness or hardness smaller than 5 mm (about 0.25 in) is considered normal for a person not exposed or infected with TB.
Patch test sites should be normal or only slightly red.
T cell responsiveness tests should be positive; that is, the injected areas should be reddened and hard. Two affected areas of 2 mm or more is considered a positive result.
Abnormal results
TB exposure is indicated by a reaction of 10 mm or more. The degree of redness is not important. A 5–10 mm area could indicate exposure if there is an underlying risk to TB.
Patch test areas that become reddened and irritated indicate reaction to the substance in the patch.
Absence of any reaction to injected areas indicates lack of T cell responsiveness, a condition called anergy. T cell anergy is seen in immune deficiency diseases including AIDS, some cases of infectious diseases, malignancies, immunosuppressive therapy (including corticosteroid treatment), some autoimmune diseases, malnutrition, major surgery, and some viral immunizations.
BOOKS
Davies, R., and S. Ollier. Allergy: The Facts. Oxford University Press, 1989.
Lawlor Jr., G. J., et. al. Manual of Allergy and Immunology. Little, Brown and Co., 1995.
Richard Robinson
KEY TERMS
Allergen—A foreign substance that provokes an immune reaction in some sensitive people but not in most others.
Anaphylaxis—An exaggerated, life-threatening hypersensitivity reaction to a previously encountered antigen.
Antibody—An immune system protein made to fight infection.
Antigen—A foreign substance detected that provokes an immune reaction.
