In this bone marrow biopsy section at medium power, there are sheets of plasma cells of multiple myeloma that are very similar to normal plasma cells, but the cells may also be poorly differentiated. Usually, the plasma cells are differentiated enough to retain the function of immunoglobulin production. Thus, myelomas can be detected by an immunoglobulin "spike" on protein electrophoresis as shown below, or the presence of Bence-Jones proteins (light chains) in the urine. Immunoelectrophoresis characterizes the type of monoclonal immunoglobulin being produced.

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