Fig. 2: Haemagglutination with human monoclonal antibodies or nanobodies to the SARS-CoV-2 RBD.

A Titration of IH4-RBD and monoclonal antibody CR3022 to RBD. Doubling dilutions of CR3022 and IH4-RBD were prepared in separate plates. 50 µL red cells (O−ve whole blood diluted 1:40 in PBS) were added to the CR3022 plate, followed by transfer of 50 µL titrated IH4-RBD. From this titration, 100 ng/well of IH4-RBD was chosen for detection. Similar results were obtained in five other experiments, performed with three separate batches of IH4-RBD. B Detection of other anti-RBD monoclonal antibodies and ACE2-Fc. Doubling dilutions of monoclonal antibodies were prepared in duplicate in 50 µL PBS (from a stock solution of 20 µg/mL) from left to right, 50 µL of 1:40 O−ve red cells were added, followed by 50 µL of IH4-RBD (2 µg/mL in PBS). The endpoint was defined as the last dilution without teardrop formation on tilting the plate for ~30 s. The binding sites for CR3022, EY6A, and ACE2 on RBD have been defined^8,10,27,30. EW-9B and EW-9C are monoclonal antibodies against non-RBD epitopes on the spike protein ²⁸. ACE2-Fc has been described in ref. ²⁸. Similar results were obtained in two other titration experiments.