 Factor H can bind C3b much more easily in the presence of sialic acid, which is a component of most cells in the human body; conversely, in the absence of sialic acid, factor B can bind C3b more easily.This means that if C3b is bound to a "self" cell, the presence of sialic acid and the binding of factor H will prevent the complement cascade from activating; if C3b is bound to a bacterium, factor B will bind and the cascade will be set off as normal.This mechanism of immune regulation using Factor H has been exploited by several bacterial pathogens.