An affinity for complement C3 as a possible reason for the potency of naturally occurring antibodies in mediating tissue homeostasis.

Certain naturally occurring antibodies stimulate alternative complement pathway C3b deposition. We propose that only those IgG antibodies stimulate alternative complement pathway which have an affinity for C3. Their weak binding to C3 in plasma increases the probability that covalently linked C3b-IgG complexes are formed during C3 activation. Such complexes are known to be much more efficient than C3b in mediating positive feedback of C3 activation, since they are more stable against inactivation by factor I and H. The hypothesis is supported by functional properties of naturally occurring antibodies to erythrocyte band 3 protein. Their ability to stimulate alternative complement pathway C3b deposition increases the potency of these low titer antibodies as opsonins.