BCG is a –
**BCG Vaccine**
**Core Concept**
BCG (Bacillus Calmette-Guérin) is a live, attenuated vaccine used primarily against tuberculosis (TB). It is made from a weakened form of Mycobacterium bovis, a bacterium that causes TB in cattle. The vaccine is administered to protect against severe forms of TB, particularly in children.
**Why the Correct Answer is Right**
The BCG vaccine works by inducing a cell-mediated immune response, which involves the activation of T-cells and the production of cytokines. This immune response helps to control the growth of Mycobacterium tuberculosis, the bacterium that causes TB. The vaccine is particularly effective in preventing disseminated TB in children, which can be a severe and life-threatening form of the disease.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because BCG is not a killed-virus vaccine. It is a live, attenuated vaccine, which means it contains a weakened form of the bacteria.
**Option B:** This option is incorrect because BCG is not a conjugate vaccine. Conjugate vaccines are made by linking a weakened form of a bacteria to a carrier protein, which helps to boost the immune response. BCG is a live, attenuated vaccine that relies on its own ability to stimulate an immune response.
**Option C:** This option is incorrect because BCG is not a subunit vaccine. Subunit vaccines contain only a specific component of a bacteria, such as a protein or polysaccharide. BCG is a live, attenuated vaccine that contains the entire bacteria.
**Clinical Pearl / High-Yield Fact**
The BCG vaccine is not effective against pulmonary TB, which is the most common form of TB in adults. However, it can provide protection against severe forms of TB, such as TB meningitis and miliary TB.
**Correct Answer: B. Live, attenuated vaccine.