Introduction

The cell populations that compose all multicellular organisms share the need to sense their environment and survey features of their cellular neighbors. These surveillance functions serve to ensure appropriate cell sorting and cellular association during embryonic development and metamorphosis, trigger repair processes that eliminate damaged or dysfunctional cells, and detect the presence of foreign (nonself) cells and elicit appropriate responses to eliminate the intruders. Collectively, the surveillance and effector mechanisms that serve the latter repair and defense functions are referred to as the organism's immune system.

We are most familiar with the antibody-based immune systems of vertebrates. These systems are capable of efficiently detecting “foreignness”, either in the form of nonself cells, aberrant self-cells or nonself macromolecules. They also provide for specific memory of previous encounters with nonself (immunization) so that a faster and more aggressive defensive response may be mobilized in the event of a secondary encounter. It is widely believed that among all living organisms, only the small group of animals that synthesize immunoglobulins (vertebrates) are capable of mounting a classic immune response in the presence of nonself challenges.

Recently, evidence has begun to accumulate that demonstrates that a variety of nonvertebrate organisms are capable of detecting foreign cells and foreign macromolecules, of eliciting effective and specific defensive responses to these challenges, and of acquiring “primed” states in which they are capable of surviving challenges that otherwise would be lethal to a “naive” individual.