The invention relates to diagnostic methods to predict whether a subject is predisposed for acquiring a disease or to predict the therapy responsiveness of an individual patient. Provided is a method for determining whether a subject is predisposed for developing an autoimmune disease, comprising determining in a sample isolated from said subject the amount of intact genes, or gene products thereof, of the FcγRII/FcγRIII gene cluster, said gene cluster comprising the FCGR2C, FCGR3A, FCGR2A and FCGR3B genes encoding an activating FcγR, and FCGR2B encoding an inhibitory FcγR; and correlating said amount to the amount observed in a healthy population. Also provided is a method to predict the responsiveness of a subject to therapy with intravenous immunoglobulin (IVIg) therapy or a monospecific biological, such as a humanized or human monoclonal antibody or a chimeric molecule, comprising the C-terminal Fc-tail of IgG.