THE PLASMA cell dyscrasias (PCDs), of which multiple myeloma is the most common and also the prototype disease, represent a group of conditions that have as their common feature an abnormal and frequently, but not invariably, neoplastic proliferation of plasma cells.1-3 The term plasma cell is used to describe all cells capable of synthesizing and releasing all classes of immunoglobulins and their subunits, recognizing that this functional definition includes cells with some morphological diversity, variously referred to as "lymphocytoid plasma cells" and "plasmacytoid lymphocytes." As schematically shown in Fig 12-1, the unbalanced or disproportionate proliferation (ie, dyscrasia) of one or a limited number of clones of plasma cells or their B-cell precursors is characteristically associated with the elaboration of excessive quantities of a homogeneous molecular population of one of these proteins, ie, IgG, IgA, IgM, IgD, or IgE, their constituent heavy or light polypeptide components, or both (Fig 12-2).
Osserman EF, Merlini G, Butler VP. Multiple Myeloma and Related Plasma Cell Dyscrasias. JAMA. 1987;258(20):2930-2937. doi:10.1001/jama.1987.03400200136016