We are unravelling the solution structures of the Fab and Fc fragments in the IgA antibody family important in mucosal immunology. Having completed the structures for IgA monomer, dimer and its complex with secretory component, we now wish to apply our powerful constrained modelling approach based on neutron and X-ray scattering to study the abnormal form of IgA dimers that is involved in a common cause of renal failure in order to understand how this disease is caused. We are also investigating the solution structures of complexes formed between the major complement regulator factor H and its target C3b. C3 undergo large conformational changes to form C3b, and different crystal structures reveal different domain arrangements. We wish to identify the C3b solution structure by constrained modelling in order to elucidate the moelcular mechanism of how C3b is regulated by factor H.