What Is The M Protein And Why Is It Important?

What Does the M Protein Mean and Where Does it Come From?

What is the M protein?. Istockphoto.com/Stock Photo©extender01

Definition: M-Protein

In blood cancers such as myeloma, the M protein refers to part of a monoclonal protein produced by cancerous cells.  There are several different settings in which the medical term "M-protein" is used, but here we are talking primarily about the significance of the M protein in cancer.  In the image, the Y shaped blue antibody, or immunoglobulin, is the M protein. The big round object is a plasma cell.

Not all M proteins are whole antibodies. Sometimes the myeloma's M protein is only one piece of an antibody.

Other Names for M Protein

M protein can refer to different things, depending on what medical topic you are discussing. In the context of myeloma, M protein refers to the abnormal product of your body's antibody-producing plasma cells; this product may also be described as follows:

  • Monoclonal protein
  • Myeloma protein
  • Free immunoglobulin light chains
  • Paraproteins
  • Bence Jones proteins
  • The M spike

M Proteins Elsewhere in Medicine

  • M protein may stand for viral matrix protein as in the M1 protein of the influenza virus
  • M protein may be used in referring to the bacteria streptococcus pyogenes
  • M protein, or actually protein M, refers to the bacteria mycoplasma genitalia

What is the M Protein in Blood Cancer?

The M protein is an immunoglobulin or part of an immunoglobulin that is monoclonal, or produced by a single clone of problematic cells.

It is not normal for the body to have so many copies of the exact same protein as generally occurs in myeloma, and this excess is detectable in laboratory studies.

Where Does the M Protein Come From?

In multiple myeloma, the M protein comes from a great excess of plasma cells. Ordinarily, plasma cells will produce a wide range of antibodies - polyclonal antibodies or polyclonal immunoglobulins.

When plasma cells are cancerous, all of the plasma cells are the same - in other words, they are clones of the same cell, and they make only the same types of products - called a monoclonal proteins. Since there are a lot of plasma cells, they make a lot of this monoclonal protein. Usually it is a whole antibody, but sometimes it's just a piece of an antibody.

Structure of Monoclonal Antibodies 

Each antibody is made up of four parts. There are two long chains on the inside, and two shorter chains on the outside. In the image, you can see a plasma cell with an antibody. In this case, all of the antibodies in the picture are the same. That is, they are monoclonal antibodies.

The light chains, or the outside chains in the image, are also called a Bence Jones proteis or free immunoglobulin light chains. When the M protein is a light chain, it is small enough that it passes through the kidneys, 'slipping through the strainer' into the urine. So if you only look in the blood, you can miss it.

If the M protein is a whole immunoglobulin, it can be detected in the blood since it is too large to pass to the urine, however excessive buildup of such M protein in the kidney causes kidney disease.

Importance of the M Protein

There are a few reasons that the M protein is significant:

  • Diagnosis - The M protein is often found in multiple myeloma and several other conditions 
  • Kidney damage - When M proteins are big enough and plentiful enough in the case of malignancy to clog the functional units of the kidney, this can lead to kidney disease and ultimately kidney failure.
  • Depending on the characteristics of the secreted protein, other harmful effects are possible.

Conditions With an Increase in M Protein

Conditions which may result in an elevated level of M proteins on a urine test include:

  • Myeloma - A urine test will be positive for M protein in 50 to 80% of people with myeloma.
  • MGUS - Monoclonal gammopathy of undetermined significance may have an elevated level.
  • Waldenstrom's macroglobulinemia - This cancer, which involves cells which are precursors of plasma cells, may have an elevated M protein level.

Note: Many conditions can cause a monoclonal gammopathy, a spike in one protein product; not all of these conditions are cancerous.

Also Known As: monoclonal protein, monoclonal immunoglobulin, myeloma protein, M- spike


Leukemia and Lymphoma Society. Diagnosis. Accessed 03/01/16. https://www.lls.org/disease-information/myeloma/diagnosis

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