CR2 and infection: We have shown that sera with apparently no activity when assayed on CR-negative cells (Table 2 and Figure 3), or with virus-inhibitory activity when assayed on CR-negative cells in the presence of complement (Additional File 2; Figure S2), can enhance infection by several orders of magnitude on CR-positive cells in the presence of complement (both on T cells naturally expressing CR2, and on unrelated cells engineered to express CR2; Figure 2, Figure 3, Additional File 2; Figure S2).