Illustration showing a type of white blood cell called a macrophage (purple) engulfing a bacterium (green) in a process known as phagocytosis. Phagocytosis is carried out by macrophages and certain other white blood cells as part of the immune response. Once bacteria, dead or cancerous cells, cellular debris or other foreign substances are detected in the body, macrophages are signalled. Macrophages engulf the bacteria and enclose them within a fluid-filled sac known as a vacuole. Another organelle, the lysosome, then releases enzymes into the vacuole, which break down the bacterium, rendering it harmless.