The IL-1/5 receptor homologs of poxviruses were the second examples discovered, following the poxvirus TNF receptor homologs, of 'viroceptors' or virus-encoded receptor mimics that function to bind and sequester cellular ligands away from their cognate cellular receptors. The prototypic member of this family, B15R of vaccinia virus, is a secreted member of the Ig superfamily, with highest sequence similarity to the ligand-binding domain of the cellular type II IL-1 receptor. The protein plays an important role in modulating the inflammatory cascade, particularly the fever response, following virus infection.