Provided herein methods of alleviating the symptoms of multiple myeloma in a patient or subject in need thereof, methods of treating multiple myeloma in a patient or subject, and methods of preventing development of multiple myeloma in a patient at risk thereof, such as a patient with monoclonal gammopathy of undetermined significance (MGUS), which methods include administering an agent effective to treat a Mycoplasma infection. The invention also relates to a new class of antigen, MG281 protein {also referenced as Protein M), that binds to various immunoglobulins with high affinity, and their uses in purifying immunoglobulins. The invention additionally relates to using MG281 protein and analog or derivative molecules for treating autoimmune diseases. Further provided are antigens and antibodies for use in the disclosed methods, and the identification of molecules that bind to MG281 protein.