antibody-independent binding of danger signals such as bacteria, yeast and virusinfected cells, but also protein A, C-reactive protein, cobra venom factor, polysaccharides and damaged tissue (14, 16) . Because constant activation of the alternative pathway is due to spontaneous hydrolysis of the highly reactive C3, constant control by complement regulators is required (17) . Healthy cells are capable of various control mechanisms that prevent the spontaneous activation of C3 and protect the host from undesirable complement activation. These control mechanisms exist both in the fluid phase and membrane bound (see below).