Delayed hypersensitivity skin test
A delayed hypersensitivity test is an immune function test measuring the presence of activated T cells that recognize a certain substance.
The immune system protects against infection by viruses, bacteria, fungi, andparasites. After initial exposure to a foreign substance, or antigen, the immune system creates both antibodies and sensitized T cells. Both these immuneagents respond when the body is reexposed to the antigen. Antibodies, whichare 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 delayed hypersensitivity test (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 TBantigen, which contains no organisms (live or dead) but can still provoke animmune response.
- To test for allergic sensitivity to potential skinirritants, such as poison ivy. Skin allergy testing is usually done by placing a series of adhesive patches on the skin containing potential allergens, orallergy-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 beones 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).
The most accurate TB test is the Mantoux test, in which a small amount of TBantigen is injected into the skin. The area is examined 48-72 hours after theinjection.
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 performedfollowing 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 afterthe injection.
Absence of exposure to TB is indicated by absent or very little skin reaction; redness or hardness smaller than 5 mm (about a quarter of an inch) is considered normal for a person not exposed or infected with TB. 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 sites should be normal or only slightly red. Patch test areas thatbecome reddened and irritated indicate reaction to the substance in the patch.
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 considereda positive result. Absence of any reaction to injected areas indicates lack of T cell responsiveness, a condition called anergy. T cell anergy is seen inimmune 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.
No special precautions or preparations before the test are necessary for mostpatients. Those with known hypersensitivity to certain skin irritants shouldalert the clinician performing the test. Some commercial preparations of fungal antigens contain mercury, a source of irritation to some patients.
After the test, patches should be kept dry. Injection sites may be washed, but excessive rubbing should be avoided. Patches and injection sites may becomereddened or irritated. If a patch causes severe itching or discomfort, the patient should remove it immediately.
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.