Mutations and polymorphisms in the gene encoding factor H (CFH) have been associated with atypical haemolytic uraemic syndrome, dense deposit disease and age-related macular degeneration. The disease-predisposing CFH variants show a differential association with pathology that has been very useful to unravel critical events in the pathogenesis of one or other disease. In contrast, the factor H (fH)-Ile62 polymorphism confers strong protection to all three diseases. Using ELISA-based methods and surface plasmon resonance analyses, we show here that the protective fH-Ile62 variant binds more efficiently to C3b than fH-Val62 and competes better with factor B in proconvertase formation. Functional analyses demonstrate an increased cofactor activity for fH-Ile62 in the factor I-mediated cleavage of fluid phase and surface-bound C3b; however, the two fH variants show no differences in decay accelerating activity. From these data, we conclude that the protective effect of the fH-Ile62 variant is due to its better capacity to bind C3b, inhibit proconvertase formation and catalyze inactivation of fluid-phase and surface-bound C3b. This demonstration of the functional consequences of the fH-Ile62 polymorphism provides relevant insights into the complement regulatory activities of fH that will be useful in disease prediction and future development of effective therapeutics for disorders caused by complement dysregulation ; 10 páginas, 7 figuras -- PAGS nros. 3452-3461 ; This work was supported by MRC Project Grant Ref 84908 (to C.L.H. and B.P.M.), Ministerio de Ciencia e Innovación Ref SAF 2005-00913 (to S.R.deC.) the CIBER de Enfermedades Raras and Fundación Renal Iñigo Alvarez de Toledo (to S.R.deC.).