What defines Waldenström's macroglobulinemia as a malignancy?

Waldenström's macroglobulinemia is primarily defined by the overproduction of immunoglobulin M (IgM) by neoplastic plasma cells. This condition is classified as a malignancy due to the proliferation of these abnormal plasma cells, which leads to an excessive amount of IgM in the bloodstream. The elevated levels of this particular immunoglobulin are responsible for various clinical manifestations, such as hyperviscosity syndrome, which can significantly affect blood flow and lead to complications.

The presence of bone pain is not a definitive characteristic of Waldenström's macroglobulinemia compared to other hematologic malignancies. Similarly, while IgG is an immunoglobulin produced by mature plasma cells, it is not the distinguishing factor in this specific condition. Maturation of fully differentiated plasma cells pertains more to the normal physiological process of plasma cell development rather than indicating malignancy. Therefore, it is the production of IgM that distinctly highlights the pathological state of Waldenström's macroglobulinemia.