Select the word that means releases for use. The body's immune system fights off infections to help the body stay healthy. When bacteria or viruses invade, the immune system first identifies them as threats. If these invaders appear dangerous, the immune system mobilizes the appropriate forces and sends them to attack. White blood cells surround, ingest, and kill the infection. Once it is under control, the immune response ends, and most of the white blood cells self-destruct. A certain number will remain as memory cells, to be prepared for the next infection.