Figure 1.

SRSD protocol for determining apparent affinity of antibodies in patient samples. SARS-CoV-2 spike RBD antigen is titrated into aliquots of diluted patient serum/plasma and incubated to equilibrate antibody-antigen binding. These equilibria are probed with RBD-coated microparticles to sample the unbound patient antibody that remains in solution (using sparse microparticles and a short incubation time to minimise perturbation of the equilibrium). Subsequently, the microparticle-collected antibody is quantified with Cy3-labelled anti-human-IgG conjugate and fluorescence imaging. Finally, we calculate the fraction of RBD-bound antibody that remains in solution and plot it as a function of total RBD concentration.