So this is how I understand Mgus.

I had to explain this to a friend the other day.  Not sure this is right but I thought I would post it. 

The process starts when your body calls for "new" plasma cells. Your body releases a "stem cell" which goes out and finds a group of plasma cells to emulate. What it is supposed to do is follow and learn from the plasma cell and become just like it (differentiate). Sort of like an intern at your job learns to become a good employee. 

But something goes wrong. The stem cell does not quite make it / graduate. It doesn't become a full fledged normal plasma cell. It becomes a "clone" or a clonal cell.  Usually becoming an IGM, IGG, or IGA (kappa or lambda) based on what cell it tries to emulate.  These clonal cells release a protein (M-protein) which is how you know if you have an M protein you could have a clonal cell.  It is not always the case.

To me the best way to understand it is the movie "Multiplicity". In that movie Michael Keaton makes clones of himself. But each clone degrades a little bit more than the last one. Now each clone is functional (so for Mgus the body doesn't destroy the clonal cell because the cell is still functional and the body probably sees nothing wrong with it) but it is "off" and a bit hyperactive. Whereas the normal Michael Keaton reproduces with two children... the clone of Michael Keaton may have 6 kids. (all a little off because they are reproductions of a clone).  The real Michael Keaton may go to work and produce 40 widgets at his job but hyperactive clonal Michael Keaton will produce 150. 

In the same way, the clonal cell may reproduce more than normal cells. And just because you have more clonal cells, and they are a bit hyperactive, they are going to produce more of the type of antibody that they were trained to produce (in my case it is IGM kappa). And with an increase in antibodies, there will be an increase in "light chains" because light chains are essentially a bi-product of antibodies.  So because I am kappa my ratio will likely always go up.  

The clonal cells live in your bone marrow so a way of working backwards and trying to figure out how many clonal cells could be in your bone marrow is by testing the levels in your blood of the "M-Protien", "The Antibodies", and "the light chains".  These give a hint as to how aggressive the replication of the clonal cell is.  

So it seems to me that at its source...  the goal is to keep the clonal cell suppressed and from reproducing too much.  More modern treatments, immunotherapy, are looking to "tag" these clonal cells and tell the immune system to go destroy them. The immune system doesn't generally because, as mentioned, the cells are not recognized as defective. You and I know they aren't right, but they are not wrong enough to activate an immune response. 

And go watch Multiplicity if you haven't seen it. Super funny.