B19R is an interferon decoy receptor encoded by the Vaccinia virus genome and by the genomes of other orthopoxviruses. It is known as B19R in the Copenhagen strain of Vaccinia and as B18R in the Western Reserve (WR) strain. B19R functions as a decoy receptor for type I interferons (IFN alpha, beta, omega). It binds to type I interferons from multiple species and prevents IFN signaling through its receptors. B19R binds to the surface of virus infected and uninfected cells where it retains its capacity to bind and neutralize IFN. It shields those cells from the antiviral effects of type I interferons, thereby enabling virus replication and pathogenicity.