Polypeptides and other compounds that can bind specifically to the CH2-CH3 cleft of an immunoglobulin molecule, and methods for using such polypeptides and compounds to inhibit Fc-mediated immune complex formation, Immune complexed IgG to IgG FγR binding, and immune complexed IgG mC1q (membrane C1q) or soluble C1q binding. Such compounds may have therapeutic use in treating amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD), and Alzheimer's disease (AD).
