E. L. Cooper The Immunodefense System Because invertebrates are exceedingly diverse and numerous, estimates reveal nearly 2 million species classified in more than 20 phyla from unicellular organisms up to the complex, multicellular protostomes and deuterostomes. It is not surprising to find less diverse defense/immune responses whose effector mechanisms remain to be completely elucidated. Of course, I am not advocating that the few of us devoted to analyzing invertebrate immunity attempt the Herculean task of examining all these species to uncover some kind of unique response! As these two volumes will reveal, we are doing fairly well in examining in depth only the most miniscule examples of invertebrates, some of which have great effects on human populations such as edible crustaceans or insect pests. This is in striking contrast to the mass of information on the mammalian immune response which has been derived essentially from the mouse, a member of one phylum, Vertebrata, an approach, reductionist to be sure, but one that has served well both the technological and conceptual advances of immunology as a disci pline. The essential framework of immunology, the overwhelming burst of results since the 1960s, have emanated primarily from this single animal. We should not forget the thymus and the bird’s bursa of Fabricius, without which we might have been slower to recognize the bipartite T /B system.