Alternative complement pathway activation occurs in the fluid phase and is controlled by Factor I and Factor H. In the absence of either Factor I or Factor H regulators, activation is uncontrolled and proceeds continuously resulting in the consumption of C3 and Factor B. In this situation, little active C3 is present that can deposit on biological surfaces. However, at biological surface either membrane-bound or surface-attached regulators like Factor H exist to control the further progression of complement activation. In the absence of Factor H or Factor H deficiency even the low-level C3 deposition is uncontrolled and complement activation occurs e.g. at the glomerular basement membrane, and results in the deposition of complement activation products. If Factor I is absent complement is uncontrolled in the fluid phase and consumed, however, the presence of Factor H restricts further low-level complement activation at the surface of the glomerular basement membrane.