One possibility explaining this phenomenon is that in secondary infections, the virus may enter cells through the primary receptor(s) or it may also form immune complexes with preexisting nonneutralizing antibodies and interact with an alternative receptor, such as the immunoglobulin G (IgG) receptor (Fc gamma receptor, FcγR), which exists in FcγR-bearing cells including monocytes/ macrophages [54, 55]. This evidence concerns the gene FCGR2A and infection.