Provided herein are FDC-SP polypeptides and methods of using such polypeptides. Methods include, but are not limited to, altering IgA concentration in a subject, treating a subject having signs of a disorder that includes excessive IgA production, identifying a compound that decreases the concentration of IgA in an animal, and identifying a compound that treats a condition associated with increased levels of IgA. Also provided herein is an animal that has decreased expression of an endogenous FDC-SP coding sequence. The animal may develop pathophysiological features of IgA nephropathy, and/or may display increased IgA in serum, saliva, bronchoalveolar lavage fluid, or a combination thereof; increased IgA expressing B lymphocytes in circulation, lymphoid tissue, or a combination thereof; or increased IgA production in vitro by isolated B lymphocytes.