Bence Jones protein
Meaning of Bence Jones protein: Free antibody light chains spilling into the urine, and the reason a reassuring dipstick reading can sit on top of heavy protein loss.
Definition of Bence Jones protein
Bence Jones protein is free immunoglobulin light chain found in urine, produced in excess by a single abnormal clone of plasma cells. Antibodies are normally assembled from heavy and light chains in balance; when a clone overproduces light chains, the surplus is small enough to slip through the kidney filter and appear in urine. Every molecule from that clone is identical.
Why does a urine dipstick miss it?
The protein pad on a dipstick is built to detect albumin and responds poorly to anything else. Light chains are not albumin, so urine loaded with them can read as trace or negative. That mismatch is itself informative: a dipstick showing almost nothing beside a quantitative measurement showing a great deal points toward light chains rather than toward ordinary kidney leakage. Confirming them takes urine electrophoresis followed by immunofixation, which shows both that the protein came from one clone and which light chain type, kappa or lambda, it is. A blood test measuring free light chains directly has taken over much of that work. The property that first drew attention to the protein was a heat trick: the urine clouds when warmed gently and clears again when boiled.
Which conditions produce it?
| Condition | Relationship |
|---|---|
| Multiple myeloma | Commonest cause; sometimes the only abnormal protein made |
| AL amyloidosis | Light chains misfold and deposit in organs |
| Light chain deposition disease | Chains deposit without forming amyloid |
| Waldenstrom macroglobulinemia | Accompanies a whole-antibody IgM clone |
| Monoclonal gammopathy of unknown significance | Small amounts with no organ damage |
The general name for an abnormal protein from one clone is the M protein, and a Bence Jones protein is the free light chain form of it. Its presence does not confirm multiple myeloma and its absence does not rule the disease out, since many myelomas secrete a complete antibody with little spare light chain.
What does it do to the kidney?
Free light chains cross the filter easily and are then taken up and broken down by the tubules downstream. Past a certain load that machinery is swamped, and the chains meet a protein the tubule secretes normally, clumping with it into waxy casts that plug the tubule from inside. The resulting cast nephropathy is why kidney failure so often accompanies myeloma and can be its first visible sign. Dehydration, infection, imaging contrast agents and some anti-inflammatory drugs worsen the process by concentrating the urine or slowing flow through the kidney.
Also known as
Used in a sentence
Urine immunofixation identified Bence Jones protein of kappa type.
Good to know
Standard urine dipstick pads respond mainly to albumin and largely miss free light chains, so a near-negative dipstick alongside heavy protein measured by a quantitative method is itself a recognized clue.