Metazoan species, from sponges to insects and mammals, possess successful defence systems against their pathogens and parasites. The evolutionary origins of these diverse systems are beginning to be more comprehensively investigated and mapped out. We have collected 1811 metazoan immunity genes from literature and gene ontology annotations. Tentative orthologs of these genes were identified using reciprocal protein-protein Blast searches against proteins from the GenBank and RefSeq databases. We have defined different levels or classes of ortholog group according to the order of reciprocal ortholog pairs among the seed immunity genes. The genes were clustered into these different ortholog groups. Initial phylogenetic analysis of these ortholog groups suggests that by this approach, we can collect a spectrum of immunity genes representing well the taxa in which they appear. All the immunity genes and their evidence of immune function, orthologs and ortholog groups have been combined into an open access database -- ImmunomeBase, which is publicly available from (http://bioinf.uta.fi/ImmunomeBase).