In addition to the common receptor/co-receptor-dependent mechanism of cellular attachment and membrane fusion, some viruses utilize antiviral antibodies for their efficient entry into target cells (Takada and Kawaoka, 2003) . This mechanism is known as antibody-dependent enhancement (ADE) of viral infection. Filoviruses utilize virus-specific antibodies for their entry into cells in vitro through interaction between anti-GP antibodies and the cellular Fc receptor (FcR) or complement component C1q and its ligand, which likely promotes viral attachment to cells (Takada et al., , 2003a (Takada et al., , 2007 Nakayama et al., 2011) (Figure 6) . FcR are expressed exclusively on the cells of the immune system such as monocytes/macrophages, neutrophils, B-cells, and granulocytes (Fanger and Guyre, 1992) , whereas C1q ligands have been identified in most mammalian cells (Eggleton et al., 1998; Nicholson-Weller and Klickstein, 1999) , suggesting a ubiquitous mechanism for ADE of filovirus infection.