Final answer:
Delayed hypersensitivity is a cell-mediated immune reaction occurring days after exposure to an antigen, wherein T cells and cytokines cause tissue inflammation, similar to reactions observed in the tuberculin skin test for TB.
Step-by-step explanation:
Hypersensitivity to substances such as tobacco extract can occur in patients who have been previously sensitized to certain antigens. When these substances are introduced into the body via intradermal injections, it can result in a delayed hypersensitivity reaction. This reaction is a type IV hypersensitivity, also known as cell-mediated immunity, which can take one to two days to reach the maximal reaction. The reaction involves T cells and the release of certain cytokines, leading to an inflammatory response in the tissue. This is similar to the mechanism seen in the tuberculin skin test for tuberculosis (TB).
During a tuberculin skin test, bacterial proteins from Mycobacterium tuberculosis are injected into the skin. A positive reaction, which manifests as raised red area called an induration due to the cellular infiltrate of activated macrophages, indicates that a patient has been exposed to TB bacteria and exhibits a cellular immune response to it. This test is an example of delayed hypersensitivity, as the reaction appears a couple of days after the injection. Intradermal testing not only helps to diagnose type I hypersensitivities through observation of immediate wheal-flare reactions, but also assists in identifying contact dermatitis and delayed-type hypersensitivity, like in the case of tuberculosis. The application of cortisone can reduce this type of immune response, as it inhibits the production of cytokines involved in inflammation.