Final answer:
A large, red welt from a tuberculin skin test indicates a positive response, meaning past exposure to TB or a prior BCG vaccination. The reaction is caused by an immune response involving activated macrophages. Further testing may be necessary to confirm active tuberculosis infection.
Step-by-step explanation:
If your tuberculin skin test results in a large, red welt, it means that the test is positive. This indicates that you have been exposed to Mycobacterium tuberculosis, the bacterium that causes tuberculosis (TB), and your body has mounted a cellular immune response to it.
The redness and induration (hardened area) you see is due to the accumulation of activated macrophages which is part of your body's immune response to the bacteria. The Mantoux tuberculin skin test is a standard method used to detect previous exposure to TB, but it's important to note that a positive test doesn't necessarily mean that there is an active infection.
A past exposure or vaccination with the BCG vaccine can also result in a positive result, and other diagnostic measures, such as a chest radiograph, may be needed to confirm active TB infection.