Final answer:
The tuberculin skin test involves a subcutaneous injection of tuberculin protein derivative to detect exposure to Mycobacterium tuberculosis, with a positive result indicated by a raised red area known as induration.
Step-by-step explanation:
The test that applies a purified protein derivative subcutaneously to indicate whether exposure to that pathogen has occurred is a tuberculin skin test. This test involves injecting a small amount of tuberculin protein derivative into the skin and is primarily used for detecting exposure to Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB). A positive result, characterized by induration—a raised red area that is hard to the touch—suggests that the patient has been exposed to the bacterium and has a cellular immune response to it. This immune response is a result of the cellular infiltrate, which includes an accumulation of activated macrophages. It is important to note that a positive tuberculin skin test does not necessarily indicate an active TB infection, as it may reflect past exposure or latent infection.