The echinoderm immune system. Characters shared with vertebrate immune systems and characters arising later in deuterostome phylogeny.

In summary, the characters of the echinoderm immune system that we review here can be considered to illuminate the baseline nonadaptive immune systems that were our original deuterostome heritage. We still retain--and greatly rely upon--similarly functioning, nonadaptive cellular defense systems. It is worth stressing that sea urchins are long lived, normally healthy animals that display remarkable abilities to heal wounds and combat major infections. From an external point of view, their immune systems obviously work very well. Thus, their cellular defense systems are extremely sensitive, and they respond rapidly to minor perturbations, all without any specific adaptive capabilities. These systems probably function through the transduction of signals conveying information on injury and infection, just as do the equivalent systems that underlie and back up our own adaptive immune systems, and that provide the initial series of defenses against pathogenic invasions. Many extremely interesting questions remain regarding the evolution of the deuterostome immune response. Are the echinoderm and tunicate systems the same, or have the protochordates augmented the basic phagocyte system with an as yet unidentified chordate-like character? Do the jawless fishes produce Igs that would make them similar to the sharks, or are they vertebrates without an Ig system that essentially rely on an invertebrate-like, nonspecific, activated phagocyte type of immune system? How do sharks regulate their immune system without T cells and MHC class I? How do they avoid producing autoantibodies? Future research will not only answer these questions, but those answers will also be enlightening with regard to the origins of the mammalian immune system in which ancient functions and subsystems remain.