ABSTRACT
The immune system is an indispensable organ system with the capacity to distinguish between self and nonself (Fig. 1). Containing various cells, tissues, and organs, this system protects organisms against potentially harmful pathogens from the external environment as well as threats from within, such as neoplastic cells. The immune system shares at least two characteristics with the nervous system. Young individuals are born with a certain potential to learn and to react to numerous and varied environmental stimuli; both systems can learn. Once information is learned by the immune and nervous systems, it becomes in a sense imprinted, and each system retains the information in varying degrees, a process defined in both systems as memory.
