CD4 and infection: 2 With typical procedures and HIV-1 stock concentrations (e.g. 1 μg/ml p24 equivalents), the frequency of productively HIV-infected primary CD4+ T cell blasts remains relatively low at day 3 post-infection e.g., generally less than 25% of the cells exhibit intracellular p24 or CD4 down-modulation (Killian MS, independent observation).