The tick-over theory was first introduced in the 1970s to explain the presence of the initial C3b molecules, which are able to trigger complement activation by the alternative pathway in human plasma under physiological conditions. After the identification of the thioester, the predominant hypothesis has been that this bond is hydrolyzed at a slow but constant rate by nucleophilic attack by H2O, leading to the generation of C3(H2O). Here we put forward the hypothesis that the rate of hydrolysis of C3 to C3(H2O) may be greatly accelerated by the interaction between C3 and a number of biological and artificial interfaces, including gas bubbles, biomaterial surfaces and different lipid surfaces and complexes. We therefore propose that C3 should preferentially be regarded as a contact activated protein rather than a target for passive, random hydrolysis in the fluid phase. (C) 2012 Elsevier GmbH. All rights reserved.