Monoclonal B-cell lymphocytosis (MBL) is a condition in which a person has an excessive amount of abnormal B cells, which are a type of white blood cell known as lymphocytes.

The number of abnormal B cells is not as high as it would be in someone with chronic lymphocytic leukemia (CLL), and MBL is not a type of cancer. However, MBL may be a precursor to CLL in some cases.

A person with MBL may not require treatment.

However, they may need to undergo regular blood testing so that doctors can monitor their lymphocyte count. This is because some people with MBL can develop other B-cell diseases, such as CLL.

This article looks at what MBL is and the symptoms, causes, related conditions, and risk factors of the condition. It also discusses diagnosis, treatment, and outlook.

a person is lifting a vial of blood out of a holderShare on Pinterest
Ani Dimi/Stocksy

In someone with MBL, the blood contains an excess of abnormal B cells, which are a type of white blood cell known as lymphocytes.

Lymphocytes are an important part of the immune system. There are two main types of lymphocytes: B cells and T cells.

B cells produce antibodies that the immune system uses to attack viruses, toxins, and bacteria that invade the body. T cells destroy cells in the body that viruses or cancer have already taken over.

Someone can develop MBL when an abnormal B-cell lymphocyte clones itself repeatedly, making too many abnormal lymphocytes.

These abnormal B cells then continue to clone themselves, producing even more of the abnormal cells.

Although B cells typically help the immune system fight invading antigens, the lymphocytes that are clones of the abnormal B cell do not function correctly and do not perform a helpful function.

However, because a person with MBL still maintains healthy B cells, the abnormal B cells do not typically cause any ill effects either.

MBL does not usually cause symptoms. However, if the condition progresses to a B-cell disease, such as CLL, a person can experience symptoms that may include:

MBL occurs when an abnormal B cell clones itself to create more abnormal B cells, which in turn clone themselves. This results in a high number of abnormal B cells in the blood.

The high number of abnormal lymphocytes can lead to two types of MBL. These are low-count MBL and high-count MBL.

In low-count MBL, the number of abnormal B cells in the blood is lower than 5,000 per microliter (µL) of blood. This type of MBL does not typically progress to CLL and is an immunological condition rather than a precursor to cancer.

In high-count MBL, the number of abnormal B cells in the blood is higher than 5,000 µL of blood. This type of MBL shares certain clinical and biological features with CLL and progresses to CLL requiring treatment in 1–2% of cases per year.

Scientists have found that MBL has a link to CLL, and that high-count MBL, in particular, may be a precursor to CLL.

High-count MBL and CLL share various characteristics. Similarities include a certain protein that occurs in lymphocytes in people with MBL and CLL. This may be why MBL can increase a person’s risk of developing CLL.

CLL is the most common type of leukemia in adults. The cancer affects lymphocytes in the bone marrow, which then travel to the blood.

The leukemia cells can gather slowly, and a person may not experience symptoms of CLL for a few years. Over time, however, the cells can spread to other areas of the body, such as the liver, spleen, and lymph nodes.

Risk factors for MBL include:

  • age, as people over the age of 40 are more likely to develop MBL
  • sex, as females are more likely than males to develop the condition
  • family history of CLL
  • medical history, as people who have had infections such as pneumonia, hepatitis C, and cellulitis are at higher risk

A doctor may order further testing after they identify an excessive number of abnormal B cells in a patient’s blood.

This may occur after a blood test a patient has received for another reason because MBL typically does not cause symptoms. A doctor can also test to rule out the possibility of CLL.

Further tests may include:

MBL does not typically require treatment unless it progresses to CLL.

The condition does not cause symptoms. However, a doctor may request regular blood tests to monitor a person’s lymphocyte count and ensure their levels of lymphocytes do not increase further.

The outlook for MBL is positive, and the condition does not typically cause ill effects. In 1–2% of cases, high-count MBL progresses to CLL requiring treatment.

In cases where MBL does progress to CLL, the outlook is generally positive. The current five-year survival rate for CLL in the United States is 87%.

Monoclonal B-cell lymphocytosis (MBL) is a condition in which an abnormal B cell, which is a type of white blood cell known as a lymphocyte, clones itself repeatedly.

Because the cells are abnormal, they do not function effectively as part of the immune system.

There are two types of MBL: High-count MBL and low-count MBL.

MBL does not typically cause symptoms and is not a type of cancer. However, in rare cases, high-count MBL may progress to chronic lymphocytic leukemia (CLL).

MBL does not require treatment, but a person may need regular blood testing to monitor their lymphocyte levels.