CD40LG and plasmacytoma: Similarly, LPL is not the sole underlying cause of a serum IgM spike, because paraproteins of this sort can also be produced by other types of B cell lymphoma with plasmacytic differentiation potential (e.g., marginal zone B cell lymphoma, MZL) [3] or, in rare cases, by bona fide plasma cell neoplasms, such as IgM+ plasmacytoma or multiple myeloma (MM) [4].