B18R, also known as B19R in the Copenhagen strain of Vaccinia, is a type I interferon (IFN)-binding protein encoded by the B18R open reading frame in the Western Reserve strain of vaccinia virus. B18R exists in two forms, soluble and membrane-bound form and also has a broad species specificity. B18R has high affinity for human IFN-alpha and also binds rabbit, bovine, rat, pig, and mouse IFN-alpha and IFN-beta. Secreted B18R binds to uninfected and infected cells. B18R presents at the cell surface and protects cells from the antiviral state. It has shown that binding of soluble recombinant B18R protects cultured cells from IFN and allows vaccinia virus replication.