CD4 and deltaretrovirus infections: The present study showed that the proviral load in CD5+ IgM+ B cells (which are the primary target cells for BLV infection) was higher than that in CD4+ T cells, CD8+ T cells, or CD5- IgM+ B cells, both in terms of proviral load per 105 cells (Table 2) and in terms of actual copy number per ml of blood (Table 3) in all cattle with a proviral load >100 copies per 105 cells.