Requirements for beta1H globulin and C3b inactivator in the control of the alternative complement pathway in human serum.

Using beta1H-depleted and C3b inactivator (C3b1NA)-deficient sera we have investigated the regulatory roles of beta1H and C3bINA in the turnover of the alternative pathway. Spontaneous turnover of C3 and factor B occurred in both beta1H-depleted and C3bINA-deficient sera. In neither case was C3d generated. Prevention of activation could be achieved by the addition of the missing protein, but not by increasing the concentration of the remaining protein. Thus both beta1H and C3bINA must be present simultaneously to prevent spontaneous activation of the alternative pathway.