Macrophage and tuberculosis vaccine. Coloured transmission electron micrograph (TEM) of phagocytosed (engulfed) Mycobacterium bovis bacteria (purple) in a macrophage (green) white blood cell. The cell nucleus (yellow) is also seen. This is the BCG (bacillus of Calmette- Guerin) strain of the bacteria, used in the vaccination for tuberculosis (TB). The bacteria is live but attenuated (weakened). The macrophage destroys the bacteria. The vaccine primes the immune system, without causing disease, so that it responds more rapidly if infected with TB bacteria.