Serum level of factor Bb increases upon NHS activation by SARS-CoV-2 spike proteins in the presence of cells. Serum Bb concentration was measured by enzyme-linked immunosorbent assay (ELISA) after incubation of SARS-CoV-2 S1 and S2 with NHS in the presence and absence of TF1PIGAnull cells in APC-specific buffer. (A) S1 and S2 proteins significantly increased the Bb concentrations in serum when cells were present. Incubating NHS with S1 and S2 in the absence of cells did not significantly elevate serum Bb concentration from the baseline NHS level. (B) Increased Bb level in the cellular phase was completely blocked by 1 µM factor D inhibitor but not by anti-C5 antibody. All experiments were repeated 3 times. Unpaired Student t test P values indicate statistical significance (*P < .05, **P < .01, NS, not significant).