Antibody Avidity: Bivalent Antigen Binding

A biosensor surface coated with antigen holds a population of antibodies, each rendered with a hinge and two Fab arms that reach down to bind antigen sites in real time. Every antibody follows a real continuous-time Markov process with association rate k_on[Ag] and per-arm dissociation rate k_off: once one Fab arm is bound, the second can locally re-engage a neighbouring antigen molecule at rate k_reb before the first arm has a chance to fully let go. Toggle between a bivalent IgG and a monovalent Fab fragment, switch between Inject and Wash phases, and watch the live-measured effective off-rate confirm the closed-form eigenvalue prediction for how avidity turns a weak per-arm bond into a functionally tight interaction.