Figure 1.

Virus neutralization during systemic delivery. (A) Circulating antibodies and (B) complement proteins bind to virus and neutralize them, as well as marking them for destruction by immune effector cells. (C) Intravenously administered virus also interacts with circulating blood cells, leading to virus sequestration. (D) Liver macrophages, which are part of the reticulo-endothelial system, filter viruses from the blood. (E) Viruses that do reach the tumor encounter extensive tumor extracellular matrix and high interstitial fluid pressure which limits their extravasation into the tumor.