Complement comprises key proteins in innate immunity that forms a major defence and clearance system in blood. CFH is the major regulator of active C3b formed from C3. C3d is a key physiological fragment of C3b that binds to both CFH and host cell surfaces. C3d thus provides extra CFH binding during inflammation. Two recent high profile publications suggested that either one or two C3d binds to the C-terminus of one CFH molecule. We will identify the correct stoichiometry and the solution structures of C3d binding to the C-terminal SCR-19/20 fragment of CFH by neutron scattering, deuterating either C3d or SCR-19/20, and studying the complexes in mixtures with its protonated partner. Contrast variation methods in H2O/D2O mixtures will be used. A knowledge of which stoichiometry is correct is essential to understand how C3d modulates the immune responses of CFH.