Paraproteins

Paraproteins, also called M-proteins or M-components, are abnormal proteins produced by a single clone of plasma cells or, less commonly, B lymphocytes. They are typically immunoglobulins or fragments of immunoglobulins that appear in serum or urine as a monoclonal band on electrophoretic testing.

Paraproteins can consist of intact immunoglobulins (for example IgG, IgA, or IgM) or free light chains (kappa or lambda). In some cases only parts of an immunoglobulin or heavy-chain–only proteins are produced. The presence of a monoclonal protein indicates a clonal proliferative disorder of B-lineage cells or plasma cells, ranging from indolent to malignant.

Diagnosis relies on laboratory testing. Serum protein electrophoresis (SPEP) detects an M-spike, which is then characterized by immunofixation electrophoresis to determine the type of immunoglobulin and light chain involved. Quantitative free light-chain assays assess kappa and lambda chains. Urine tests can detect Bence Jones proteins. Bone marrow examination may be performed to assess plasma cell burden in disorders such as multiple myeloma or plasmacytoma.

Clinical relevance varies. Paraproteinemia occurs in disorders including monoclonal gammopathy of undetermined significance (MGUS), multiple myeloma, Waldenström macroglobulinemia (often IgM paraprotein with hyperviscosity), and light-chain–producing diseases. Complications can include kidney damage (myeloma kidney), AL amyloidosis from light chains, hyperviscosity in IgM diseases, and end-organ damage in myeloma.

Management is disease-specific, ranging from observation with regular monitoring in MGUS to systemic therapy for myeloma or Waldenström, involving chemotherapy, targeted therapies, immunotherapy, or stem cell transplantation, along with supportive care for renal, hematologic, and infectious complications.