The immune system is one of the most important means by which more complex animals protect themselves from external threats. There are two immune systems: the innate and the adaptive. The innate is the most basic and has existed for the longest time; the adaptive immune system is a more recent evolutionary development. The immune system plays a critical role in surveillance and prevention of malignancy. It is only when malignant cells develop mechanisms to escape the immune system that they become clinically significant tumors. Our knowledge of the immune system has grown enormously. We have begun to understand not only the mechanisms by which it protects us but also the ways in which it can inflict injury on the body. We are also learning about how it communicates with the environment and how its various components interact within the body. With this information, we are learning how to manipulate it to our greater benefit.