WM is a cancer of the lymphatic system which occurs in a type of white blood cell called a B-lymphocyte or B-cell. B-cells normally mature into plasma cells - whose job is to manufacture immunoglobulins (antibodies) to help the body fight infection. In WM, there is a malignant change to the B-cell in the late stages of maturing, and it continues to proliferate into a clone of identical cells, primarily in the bone marrow but also in the lymph nodes and other tissues and organs of the lymphatic system. These clonal cells over-produce an antibody of a specific class called Immunoglobulin M (IgM).
You can find the subtype review on Waldenström’s Macroglobulinemia here.