Macrophage engulfing tuberculosis vaccine. Coloured scanning electron micrograph (SEM) of a macrophage white blood cell engulfing Mycobacterium bovis bacteria (yellow). 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 engulfs (phagocytoses) the bacteria and destroys them. The vaccine primes the immune system, without causing disease, so that it responds more rapidly if infected with TB bacteria. Magnification: ×2, 900 when printed 10 centimetres wide / SCIENCE PHOTO LIBRARY.