Frogs, fish, and humans each have extensive, differentiated lymphatic tissues, display major histocompatibility complex class I and class II molecules on all cells, and have B cells that produce antibodies with a vast range of antigen-binding specificities (Du Pasquier, 1992). This homogeneity has been a boon for experimental immunology, allowing it to successfully use rat and mouse models to illuminate the human immune system. The evolutionary conservatism of the vertebrate immune system is a testament to its plasticity, the elegance of its design, and its robustness in functioning in the world"s diverse disease ecologies. Why is this the case? Why is the immune system so highly conserved, while other physiological systems are more specifically adapted to our mammalian ancestry or hominid roots?