Final answer:
A positive PPD test means that the individual has had contact with Mycobacterium tuberculosis and their immune system has mounted a cellular response, but it does not definitively indicate active TB.
Step-by-step explanation:
A positive reaction to the PPD (Purified Protein Derivative) test, also known as a tuberculin skin test, is interpreted as the individual having had contact with Mycobacterium tuberculosis. This test indicates that the person's immune system has responded to the bacterial proteins injected into the skin through a delayed hypersensitivity reaction, characterized by induration, a raised, red, and hard area. This immune response involves activated macrophages and the accumulation of cellular infiltrates. However, it does not necessarily mean that the individual has active tuberculosis (TB); they could have been exposed to TB in the past, the infection could be latent, or they could have received the BCG vaccine which may result in a false-positive due to previous exposure to similar antigens.