Antibodies, dissolved in blood , lymph , and other body fluids bind the antigen and trigger a response to it. A tiny amount of protein, extracted from the bacteria, is injected into the skin. If the subject is currently infected, or has ever been infected , with the bacteria, a positive test results. In 24 hours or so, a hard, red nodule develops at the site of the injection. This nodule is densely packed with lymphocytes and macrophages .
Many people develop rashes on their skin following contact with certain chemicals.
Nickel, certain dyes, and the active ingredient of the poison ivy plant are common examples. The actual antigen is probably created by the binding of the chemical to proteins in the skin. These microorganisms are engulfed by phagocytic cells , like macrophages, but evade the normal intracellular mechanisms that should destroy them. However, the macrophages can present fragments of antigens derived from these parasites. These are displayed in the class II histocompatibility molecules of the macrophages.