Monoclonal gammopathies represent a wide spectrum of related diseases.     The common denominator is the presence of a monoclonal protein in the serum or urine, which can be in the form of intact immunoglobulin, immunoglobulin fragments, and/or free light chains. This will be accompanied by the presence of monoclonal plasma cells in the bone marrow, in soft tissue (with plasmacytomas), or in the peripheral circulation (in more advanced disease stages).
Plasma cells are terminally differentiated (specialized cells unable to proliferate) effector cells of B-cell lineage.     They are the primary mediators of humoral immunity, secreting antigen-specific immunoglobulins. Abnormalities of plasma cells are responsible for a variety of autoimmune diseases and plasma cell neoplasms. Clonal evolution of one or more plasma cells sets the stage for development of the monoclonal gammopathies.    
Plasma cells normally secrete an intact immunoglobulin that is made up of 2 identical light chains and 2 identical heavy chains. There are 5 major classes of heavy chains, which correspond to the major classes of immunoglobulins: mu (IgM), delta (IgD), gamma (IgG), alpha (IgA), and IgE (epsilon). In each of the immunoglobulin molecules, the heavy chains are bound to one or the other of the 2 light chains (either kappa or lambda), but not both. The heavy chains, which have 4 or 5 domains, and the light chains, which have 2 domains, are covalently bonded to each other through disulfide bonds.
Monoclonal proteins are abnormal, immunologically homogeneous immunoglobulins or parts of immunoglobulins, produced by a single clone of plasma cells. They may be the result of an underlying lymphoid malignancy, be part of a clonal expansion of plasma cells causing no symptoms (e.g., monoclonal gammopathy of unknown significance), or lead to life-threatening complications (e.g., primary amyloidosis). 
Protein electrophoresis is performed to detect and identify monoclonal proteins in the serum and urine.   Quantitation of immunoglobulins is achieved by nephelometry. Immunoglobulin free light chains in serum are assessed using antibodies specific for the light chain part of the immunoglobulins.
Clonal expansion of plasma cells is the underlying abnormality among the monoclonal gammopathies. These cells may be found in the bone marrow, peripheral circulation, or soft tissue. They are typically demonstrated on bone marrow exams, where presence of clonal plasma cells may or may not be accompanied by an absolute increase in the plasma cell proportion. Whereas plasma cells are identified by their surface staining for CD138 (syndecan) on immunohistochemistry, demonstration of clonality depends on light chain restriction, excess of kappa- or lambda-expressing plasma cells resulting in a skewing of the normal kappa to lambda ratio.  The skewed ratio can be demonstrated by immunohistochemistry using antibodies against kappa or lambda light chains or by polymerase chain reaction (PCR) performed on marrow biopsy specimens.
Clonal plasma cells numbers are typically small in most monoclonal gammopathies, except for advanced myeloma and plasma cell leukemia.   Sensitive flow cytometry techniques can be used for routine evaluation of bone marrow aspirates to detect small numbers and for immunophenotypic characterization of abnormal plasma cells.   
Monoclonal gammopathy of undetermined significance (MGUS) is the most common monoclonal gammopathy and is found in about 1% to 2% of adults.       The prevalence increases with age and is generally higher in patients over 70 years of age. Prevalence rates vary according to geographic and racial factors, with lower rates in Asia compared with those in Europe and North and South America, and higher prevalence rates among black people compared with white people.
There are disease conditions other than MGUS where a monoclonal protein may be detected in the serum and/or urine. These include:
Lymphoproliferative diseases where clonal B lineage cells secrete a monoclonal protein (chronic lymphocytic leukemia, non-Hodgkin lymphoma, post-transplant monoclonal gammopathies  )
Conditions associated with or predisposing to a higher prevalence of monoclonal gammopathy (hepatitis C virus infection, HIV infection)
Infectious or inflammatory conditions associated with a transient development of several clones of reactive B cell/plasma cell populations (systemic lupus erythematosus, rheumatoid arthritis, psoriatic arthritis, Sjogren syndrome, Schnitzler syndrome).
Professor of Medicine
Division of Hematology
SK serves on advisory boards for Takeda, Celgene, Janssen, AbbVie, and Kite. He has been principal investigator on clinical trials supported by Takeda, Kite, Celgene, Janssen, AbbVie, Sanofi, and MedImmune.
University of Turin
AP declares that he has no competing interests.
Price Eminent Scholar and Professor of Medicine Director
Bone Marrow Transplant Program
Division of Hematology and Oncology
University of Florida College of Medicine
JRW declares that he has no competing interests.
Associate Professor of Clinical Medicine
Department of Medicine
Division of Hematology/Oncology
University of Virginia
JD declares that he has no competing interests.
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