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