Monoclonal B-cell lymphocytosis is when your body produces an elevated number of identical B cells. In some cases, it can develop into chronic lymphocytic leukemia.

Monoclonal B-cell lymphocytosis (MBL) is estimated to affect about 5–12% of people. It doesn’t cause symptoms and doesn’t require active treatment. A subtype called high-count MBL can transform into a type of blood cancer called chronic lymphocytic leukemia (CLL).

B cells are a type of white blood cell that produces antibodies. Antibodies are proteins that tell other immune cells to destroy potentially harmful foreign substances like viruses or bacteria.

Normally, B cells clone themselves when they’re activated by cells they perceive as harmful. In people with MBL, genetic changes inside B cells cause them to produce too many clones of themselves that don’t function properly.

This article looks at MBL in more depth, including risk factors, causes, and the outlook for people with this condition.

The name “monoclonal B-cell lymphocytosis” can be broken into three parts:

  • Monoclonal: Monoclonal refers to cells that originate from a single cell.
  • B cell: B cell refers to a type of white blood cell that produces antibodies.
  • Lymphocytosis: Lymphocytosis is an increased count of lymphocytes, a category of white blood cells that include B cells.

MBL is an elevated number of identical B cells with a total B-cell count under 5,000 per microliter of blood for at least 3 months and no other evidence of blood cancer such as:

MBL doesn’t usually cause symptoms, but a doctor may notice abnormalities in your blood cell counts when performing a blood test for an unrelated condition.

High- and low-count MBLs

Doctors often divide MBL into:

  • High-count MBL: Your MBL count is high if your total B-cell count is equal to or over 500 per microliter of blood.
  • Low-count MBL: Your MBL count is low if your total B-cell count is equal to or lower than 500 per microliter of blood.

High-count MBL is considered a precursor to CLL. MBL is considered to have progressed to CLL if your total B-cell count becomes higher than 5,000 per microliter of blood. High-count MBL only makes up about 2% of MBLs.

Is monoclonal B-cell lymphocytosis the same as chronic lymphocytic leukemia (CLL)?

MBL is considered a precursor to a type of leukemia called CLL. Many people with MBL never develop CLL. Doctors often recommend performing regular blood tests for people with high-count MBL to look for changes.

Although MBL often doesn’t progress to CLL, CLL progresses from MBL in nearly all the people who have it.

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MBL is very common and is seen in about 5–12% of people in the general population. It’s caused by genetic mutations in B cells that cause them to over-replicate themselves. Researchers don’t know exactly why this occurs, but they have identified some risk factors that may make you more likely to develop MBL.


MBL becomes much more common with age. It’s very rare in people younger than 40 years old. It’s detected in more than 20% of people ages 70 years old and older and more than 75% of people ages 90 years old and older.

Family history

A family history of cancer has been highly connected to the development of CLL. Research suggests that 13% to 18% of people develop MBL if they have at least two family members with CLL.

A family history is also the strongest risk factor for the development of CLL. The risk of developing CLL is about 8.5 times higher if you have a first-degree relative with CLL. A first-degree relative can be a:

  • parent
  • full sibling
  • child

Biological sex

MBL seems to develop more often in males than in females.

In a 2021 study, researchers found that 30% of males and 18% of females in a group of people with a family history of CLL developed MBL after a median follow-up of 8.1 years.

Language matters

You’ll notice that the language used to share stats and other data points in this article is pretty binary, fluctuating between the use of “male” and “female.” Although we typically avoid language like this, specificity is key when reporting on research participants and findings.

Unfortunately, the studies and surveys referenced in this article did not include data on, or include, participants who were transgender, nonbinary, gender nonconforming, genderqueer, agender, or genderless.

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Certain infections may be linked to MBL, especially when combined with other risk factors.

There seems to be a high frequency of MBL among people with hepatitis C infections and a low frequency of MBL among people vaccinated for pneumococcal or influenza infections.

The odds of high-count MBL progressing to CLL that requires treatment is about 1–5% per year. Less commonly, it can develop into other types of blood cancer.

MBL doesn’t cause symptoms, but early symptoms of CLL can include:

Active treatment isn’t needed for MBL, but a doctor will likely want to perform regular blood tests if you have high-count MBL to make sure it doesn’t progress.

If your cancer does progress to CLL, standard treatment options include:

Low-count MBL is common. Most people live full lives without complications. Only about 1.8% of people with it progress to high-count MBL. People with low-count MBL may be at an increased risk of severe infections.

High-count MBL usually requires regular monitoring to make sure it doesn’t transform into CLL or other cancers.

People with CLL live at least 5 years about 88% as often as people without CLL in the United States, based on statistics from 2013–2019.

MBL is when you have a high number of identical B cells in your blood. Doctors further classify it into low-count and high-count MBL.

Low-count MBL usually doesn’t progress and doesn’t cause symptoms. Doctors often want to monitor high-count MBL regularly to make sure it doesn’t progress to CLL.