Antibodies to West Nile virus: a double-edged sword

Antibodies, although critical for host defense against West Nile virus (WNV), can both neutralize and enhance viral infection. In this issue of Cell Host & Microbe, Pierson et al. dissect these opposing effects and demonstrate that, when 120 epitopes are available per WNV virion, approximately 25 are occupied by antibody at 50% neutralization. At lower occupancies, enhancement of infection dominates; at higher ones, neutralization ensues. These results are important for WNV vaccine design and for potential therapeutic use of antibodies to WNV.