TNF and infection: Infected monocytes/macrophages are present in lower quantities early in infection, and are detected in the blood later, possibly acting as virus-transporting cells to different tissues; this could also explain the increasing levels of IL-6 and TNF-α in the sera of infected mice, after day 4 p.i. These results are relevant to natural human infection since both B cells and monocytes/macrophages express Fc receptors on their surface [39] and consequently they are potential targets for antibody-dependent enhancement of infection [40].