A possible reason for any increased sensitivity of primary vs. pseudo viruses for identifying patients with potent neutralizing Abs could be the higher number of envelope glycoprotein spikes on the primary viruses relative to the PV [20, 28] . An alternative factor might be the density of (co)receptors on target cells, which has been implied by Corti et al who reported potent neutralization by HK20 in the HOS assays but almost no potency in TZMbl assays [2, 3] . Since HK20 recognizes an epitope in the gp41 region this could partially be explained by the high level of CCR5 expression on the TZMbl cells making it more difficult for anti-gp41 Abs to be effective [2, 29] . Also, the pathway employed by PV to enter TZMbl cells may be relevant so that HK20 could have been hindered by events following uptake into an endosome [30] .