The PPD (Purified Protein Derivative) test is a diagnostic method used to detect the presence of tuberculosis (TB), an infectious disease often referred to as consumption or white plague. This test identifies specific immune reactions in the blood, indicating prior exposure to TB bacteria. It involves injecting a small amount of PPD into the inner forearm. If an individual has been exposed to TB, their body develops a hypersensitivity to the protein derivative, leading to a localized skin reaction (redness and swelling) within 48-72 hours. This reaction helps confirm a potential TB infection. The PPD test is highly recommended for individuals with compromised immune systems (e.g., due to cancer, AIDS, or steroid use) and those who have had contact with TB patients, facilitating early diagnosis and timely intervention.