It is commonly acknowledged that the nervous system and the immune system, those most complex of networks, share attributes beyond their intricacy. Elements common to the two systems include memory, connectivity, flexibility and developmental selection of cellular composition by a rigorous process involving widespread programmed cell death. There is one salient difference: the cells of the immune system are predominantly in constant motion, while post-mitotic neurons and glia are largely fixed in place. Therefore, chemokines, initially characterized as leukocyte chemoattractants, have for the...