Skin test for immunity: A method of evaluating whether a person has developed an immune response to certain infections. A substance is injected into the deep layer of the skin (dermis), which will cause a reaction if the immune system recognizes it. One of the most common skin tests is the tuberculin test, which reveals whether a person has been exposed to tuberculosis. The injection is placed on the inside of the forearm. A positive reaction results in a firm area of skin within two days.