Factor H is a member of the regulators of complement activation family and is a complement control protein. It is a large , soluble glycoprotein that circulates in human plasma . Its principal function is to regulate the alternative pathway of the complement system, ensuring that the complement system is directed towards pathogens or other dangerous material and does not damage host tissue. Factor H regulates complement activation on self cells and surfaces by possessing both cofactor activity for the Factor I mediated C3b cleavage, and decay accelerating activity against the alternative pathway C3-convertase, C3bBb. Factor H exerts its protective action on self cells and self surfaces but not on the surfaces of bacteria or viruses. There are however, important exceptions, such as for example the bacterial pathogen, Neisseria meningitidis . This human pathogen has evolved mechanisms to recruit human FH and down-regulate the alternative pathway. Binding of FH permits the bacteria to proliferate in the bloodstream and cause disease.