AbstractThe C3b‐binding receptor, CR1/CD35, supports CR2/CD21‐mediated activation of complement by human B lymphocytes, possibly by associating with CR2 to promote or stabilize the binding of hydrolyzed C3 (C3i), the primary component of the AP convertase, C3i‐Bb. To evaluate this hypothesis, we examined the uptake kinetics and binding equilibria for C3i dimer interaction with human blood cells in the absence and presence of CR1‐ and CR2‐blocking mAb. C3i displayed dual uptake kinetics to B lymphocytes, comprising of rapid binding to CR1 and slower binding to CR2. The forward rate constants (k1) for CR1 and CR2, operating independently, differed ca. 9‐fold (k1=193±9.4 and 22.2±6.0×103 M–1s–1, respectively). Equilibrium binding of C3i to B lymphocytes was also complex, varying in strength by ca. 13‐fold over the C3i concentration range examined. The maximum association constant (Ka, max=109±27.2×107 l/mole) was ca. 9‐ and 6‐fold greater, respectively, than those for CR1 or CR2 acting alone (Ka=13.2±5.3 and 18.5±3.5×107 l/mole). The high avidity of the CR1‐CR2 complex for C3i is consistent with its rates of C3i uptake and release being determined by CR1 and CR2, respectively.