M protein
Meaning of M protein: A single antibody churned out in bulk by one clone of cells, showing up as a narrow spike on a laboratory trace.
Definition of M protein
An M protein is an abnormal antibody, or a fragment of one, produced in large amounts by a single clone of plasma cells. Because every copy is identical, it appears on protein electrophoresis as a narrow spike rather than the broad band that normal antibodies form. Its presence marks a plasma cell disorder, which may be harmless or may be myeloma.
How is it detected?
Blood serum is separated by an electric field, which spreads the proteins into bands by size and charge. Normal antibodies come from thousands of different clones, so they smear into a broad hump. A single clone produces one identical molecule, and identical molecules travel together and stack into a sharp peak. A second test, immunofixation, identifies which class of heavy chain and which type of light chain the antibody is built from. Free light chains circulating without a heavy chain partner are measured separately, and they are also what appears in urine as Bence Jones protein. Once identified, the size of the spike is tracked over time as a measure of how much of the abnormal clone is present.
What conditions produce one?
| Condition | Typical picture |
|---|---|
| Monoclonal gammopathy of undetermined significance | A small spike, no organ damage, usually found by chance |
| Smoldering myeloma | More clone, still no damage to bone, kidney or blood counts |
| Multiple myeloma | Bone destruction, anemia, kidney impairment or high calcium |
| Waldenstrom macroglobulinemia | An IgM protein from a lymphoma-like clone |
| AL amyloidosis | Light chains that fold wrongly and deposit in heart, kidney or nerves |
Why does it cause harm?
Rarely because of anything it does as an antibody, since the molecule is usually useless against infection. The damage comes from quantity and from behavior. Light chains filtered by the kidney clog and injure the tubules. A large amount of IgM thickens the blood so it flows sluggishly, causing headache, blurred vision and bleeding from mucous membranes. Misfolded light chains laid down in tissue stiffen the heart and kidneys. Meanwhile the expanding clone crowds out the plasma cells that make useful antibodies, so infections become more frequent even though total protein appears high.
Does finding one mean cancer?
Usually not. The great majority of people found to have an M protein have the smallest category, in which a modest clone sits quietly and causes nothing. It is monitored rather than treated, because a small proportion progress each year and the progression is detected by watching the size of the spike, the kidney function, the calcium level and the blood count over time.
Also known as
Used in a sentence
Serum electrophoresis showed an M protein in the gamma region, typed as IgG kappa on immunofixation.
Good to know
The same name belongs to an unrelated surface protein of Streptococcus pyogenes, so context decides the meaning; most people found to have a blood M protein have the harmless precursor state rather than myeloma.