Every organism's body is being attacked and invaded constantly by viruses and by living organisms such as bacteria, protists, fungi, and multicellular parasites. Without defenses against the damage, diseases, and infections caused by the invaders, the attacked organism would soon die. Organisms have evolved myriad defensive mechanisms to protect themselves from the invaders; vertebrates have evolved an extremely sophisticated defense system called the immune system, which is based on several types of highly specialized white blood cells that search out and destroy foreign materials that have entered the host's body. Some of the cells even supply long-term, future protection against reinvasion by a previously defeated attacker. The recognition of an invading substance (or organism) by the immune system cells occurs at the molecular level and is based on the three-dimensional shapes of particular molecules. The ability to recognize so many different types of invaders is conferred upon the immune system by a unique form of gene processing. A disease such as AIDS, which destroys the immune system, makes us realize with brutal clarity the importance of the immune system-without it we cannot live. We succumb to the hordes of attackers.