Bence Jones proteins are fragments of antibodies that spill into the urine when certain bone marrow cancers, most notably multiple myeloma, cause abnormal plasma cells to overproduce a single type of immune protein. Specifically, they are monoclonal immunoglobulin light chains, the smaller half of an antibody molecule, with a molecular weight of about 22,000 daltons. Because they’re small enough to pass through the kidneys’ filtering system, they end up in urine, where they serve as an important diagnostic marker for several serious blood disorders.
How These Proteins Form
Your immune system normally produces antibodies made of two heavy chains and two light chains locked together. Plasma cells in the bone marrow manufacture these antibodies, and any small excess of free light chains gets filtered by the kidneys and reabsorbed without issue. In diseases like multiple myeloma, a single clone of plasma cells multiplies out of control and churns out massive quantities of one specific light chain, either the kappa (κ) or lambda (λ) type. The kidneys can’t reabsorb this flood, so the excess light chains pass into the urine as Bence Jones proteins.
The key distinction is that these are monoclonal, meaning they all come from one rogue cell line and are structurally identical. A healthy person’s light chains are polyclonal, a diverse mix from many different plasma cells. When a lab runs an electrophoresis test on concentrated urine, Bence Jones proteins show up as a single sharp band rather than a broad smear, which is a telltale sign of an underlying clonal disorder.
Conditions Linked to Bence Jones Proteins
Multiple myeloma is the most common cause. About 20% of myeloma patients have detectable Bence Jones proteins in their urine at the time of diagnosis, and 60 to 80% will have them at some point during the disease. But myeloma isn’t the only possibility. These proteins also appear in two other immunoproliferative disorders:
- Light chain amyloidosis (AL amyloidosis): The excess light chains misfold and deposit as stiff, fibrous tangles called amyloid fibrils in organs like the heart, kidneys, and liver, gradually causing organ failure.
- Light chain deposition disease (LCDD): Instead of forming fibrils, the light chains deposit as granular clumps along the kidney’s basement membrane, leading to progressive kidney damage.
All three conditions share the same underlying problem: uncontrolled plasma cell growth producing too much of one light chain. The type of organ damage depends on how those specific light chains behave once they accumulate in tissues.
How Bence Jones Proteins Damage the Kidneys
Kidney failure is one of the most serious complications of multiple myeloma, and Bence Jones proteins are usually the direct cause. The damage happens through a process called cast nephropathy, sometimes referred to as “myeloma kidney.”
As large quantities of free light chains flow through the kidney’s tubules, they interact with a naturally occurring protein called Tamm-Horsfall glycoprotein. The two stick together and form solid casts that physically block the tubules in the deeper parts of the kidney, specifically beyond the loop of Henle. The higher the concentration of Bence Jones proteins, the worse the obstruction. Dehydration makes this significantly worse because it concentrates the proteins further in the tubular fluid.
Detection of monoclonal light chains in the urine is considered a prerequisite for diagnosing myeloma kidney. This is why doctors monitor urine light chain levels closely in myeloma patients, and why staying well hydrated is a practical concern for anyone with this diagnosis.
How Testing Works
Standard urine dipstick tests are unreliable for detecting Bence Jones proteins. Dipsticks are designed to pick up albumin, the most common protein in urine during kidney disease, but they have very low sensitivity for light chains. This means a “normal” dipstick result does not rule out Bence Jones proteinuria.
The standard approach requires collecting urine over a full 24-hour period to measure how much protein is being excreted daily. The sample is then run through urine protein electrophoresis (UPE), which separates proteins by their electrical charge and size on an agarose gel. If a suspicious band appears, a follow-up test called urine immunofixation electrophoresis (uIFE) uses specific antibodies to confirm whether the protein is a kappa or lambda light chain. Lab analysis can identify Bence Jones proteins at concentrations as low as 10 mg/L, though detection limits are approximate.
Even these specialized urine tests have limitations. UPE struggles with low protein levels and can be hard to interpret when there’s heavy proteinuria from other causes. Its sensitivity for newly diagnosed or relapsed cases is around 76%.
Blood Tests as an Alternative
A newer option is the serum free light chain (sFLC) assay, a blood test that measures free kappa and lambda light chains directly in the bloodstream rather than waiting for them to appear in urine. Its sensitivity is slightly higher, around 80.5% compared to 75.6% for urine-based testing, and it’s far more convenient since it requires only a blood draw. Many doctors now use serum free light chain testing alongside or even instead of 24-hour urine collection, particularly for monitoring treatment response. Both approaches have high specificity (above 95%), meaning false positives are rare.
The Unusual History Behind the Name
The protein gets its name from Henry Bence Jones, a London physician who published his findings in 1847 after studying a urine specimen from a 45-year-old grocer named Thomas Alexander McBean. McBean’s urine behaved strangely when heated: an opaque precipitate formed as the sample warmed, then dissolved when the temperature reached 75°C, only to reappear as the urine cooled. This odd heat-precipitation behavior became the original diagnostic test for the protein and remained in use for over a century. It wasn’t until 1956 that researchers Leonhard Korngold and Rose Lipari proved that Bence Jones proteins were actually immunoglobulin light chains, connecting the old bedside observation to modern immunology.