Fig 2.

Multivalent binding and unbinding steps between an HIV-1 virion (circle) and surface receptors on FDC. Free virions attach to the FDC surface and first form a single bond with rate constant α. This bond can break with rate constant k_r, releasing the virion back into solution. Once the virion is bound to the surface, the rate constant to form an additional bond is k_x, and the reverse rate constant is k_−x. Equation 2 in Materials and Methods describes the kinetics of these reactions and includes statistical factors that incorporate the number of ways that bonds can form and break.