Echinoderms share common ancestry with the chordates within the deuterostome clade. Molecular features that are shared between their immune systems and that of mammals thus illuminate the basal genetic framework on which these immune systems have been constructed during evolution. The immune effector cells of sea urchins are the coelomocytes, whose primary function is protection against invasive marine pathogens; here we identify six genes expressed in coelomocytes, homologues of which are also expressed in cells of the mammalian immune system. Three coelomocyte genes reported here encode transcription factors. These are an NFKB homologue (SpNFKB); a GATA-2/3 homologue (SpGATAc); and a runt domain factor (SpRunt-1). All three of these coelomocyte genes respond sharply to bacterial challenge: SpNFKB and SpRunt-1 genes are rapidly up-regulated, while transcripts of SpGATAc factor disappear within hours of injection of bacteria. Sham injection also activates SpNFKB and SpRunt, though with slower kinetics, but does not affect SpGATAc levels. Another gene, SpHS, encodes a protein related to the signal transduction intermediate HS1 of lymphoid cells. Two other newly discovered genes, SpSRCR1 and SpSRCR5, encode proteins featuring SRCR repeats. These genes are members of a complex family of SRCR genes all expressed specifically in coelomocytes. The SRCR repeats most closely resemble those of mammalian macrophage scavenger receptors. Remarkably, each individual sea urchin expresses a specific pattern of SRCR genes. Our results imply some shared immune functions and more generally, a shared regulatory architecture which underlies immune system gene expression in all deuterostomes. We conclude that the vertebrate immune system has evolved by inserting new genes into old gene regulatory networks dedicated to immunity.