Lymphocytosis is when there are too many lymphocytes or white blood cells. Symptoms include swollen lymph nodes. It can indicate an underlying health problem, such as an infection, stress, leukemia, or lymphoma.

Lymphocytes are a type of white blood cell that fights germs and prevents disease. Rapid increases in lymphocyte numbers can indicate an infection or underlying health condition. Transmissible causes of lymphocytosis are usually acute viral infections.

Lymphocytosis can have many possible causes, and some may be nothing to worry about. However, doctors must determine the cause of lymphocytosis to provide appropriate treatment.

This article explores lymphocytosis and its potential causes.

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Lymphocytosis is the medical name for a high lymphocyte count. These white blood cells include B lymphocytes or B cells and T lymphocytes or T cells.

These cells play an essential role in the immune system. B cells produce antibodies that destroy germs, and T cells fight cancerous and infected cells.

Lymphocytes play an essential role in the immune system and can spike during infections. This is a normal reaction, and lymphocytosis itself is not a serious condition. The spike in lymphocytes is necessary for the body to fight infections.

However, doctors will need to identify the cause of a person’s elevated lymphocyte levels because they could signify a more severe condition.

Symptoms that can indicate lymphocytosis are generalized lymphadenopathy, or swelling of the lymph nodes, and splenomegaly. Splenomegaly refers to an enlarged spleen, which may present as a dull pain in the upper left quadrant. This is largely a physical exam finding.

People may also have symptoms of the condition underlying their lymphocytosis. For example, flu could cause lymphocytosis and symptoms including a cough, muscle aches, and fever.

Lymphocytosis has various potential causes ranging from minor infections to cancers.

Viral infections

Various viral infections can cause lymphocytosis, including:

Paradoxically, COVID-19 causes low lymphocytes levels. Counts lower than 1,000 lymphocytes per microliter (μl) of blood usually correspond to severe disease.

One of the most common viral infections that can cause lymphocytosis is the Epstein-Barr Virus (EBV). In addition, EBV can cause an illness called infectious mononucleosis, which doctors also call mono.

Some symptoms of EBV infection include:

There is no treatment for EBV, but home remedies such as getting plenty of rest, drinking fluids, and taking over-the-counter medicines can relieve symptoms. People should also be sure to avoid all contact sports, as being tackled can rupture an enlarged spleen.


Stress due to severe diseases and medical conditions may cause lymphocytosis. This most commonly occurs in individuals who have heart conditions, use epinephrine, or experience seizures.

Symptoms will depend on which condition is causing the stress.

Bacterial infections

Most bacterial infections cause a high count of neutrophils, another type of white blood cell. However, some bacterial diseases, such as syphilis and cat-scratch disease (CSD), can cause lymphocytoses.

Bartonella henselae is a type of bacteria that can cause CSD. The name refers to its typical cause, a cat scratch. Symptoms of CSD include:

  • enlarged, tender lymph nodes
  • fever
  • a papule or pustule at the scratch site
  • headache
  • fatigue

In many cases, CSD is not serious and requires no medical treatment. However, doctors may prescribe antibiotic medication to people with CSD who have a weakened immune system due to HIV or another medical condition.

Chronic lymphocytic leukemia

Chronic lymphocytic leukemia (CLL) is the most common type of adult leukemia. It begins in B cells and causes an increase in abnormal lymphocytes. There are two types of CLL, one which grows slowly and another that develops rapidly.

People with CLL could have no symptoms at the time of diagnosis, but possible symptoms include:

Treatment for CLL varies depending on the stage and presence of symptoms but can involve:

Non-Hodgkin lymphoma

Non-Hodgkin lymphoma (NHL) is a cancer of white blood cells. There are many types of NHL, and only some have lymphocytosis as a characteristic, such as mantle cell lymphoma (MCL). MCL is rare and only accounts for roughly 6% of all NHL cases in the United States.

Some people have no MCL symptoms, leading to delayed diagnosis and treatment. When symptoms of MCL do occur, they can include:

  • fever
  • swelling of lymph nodes
  • weight loss
  • night sweats
  • tiredness

Doctors consider MCL challenging to treat, and the approach depends on the stage of cancer. They typically suggest active surveillance for people with a slow-growing disease and no symptoms.

Other treatment options include chemotherapy, proteasome inhibitors, and transplantation.

Doctors use a blood test to diagnose lymphocytosis. The test measures the number of lymphocytes in a person’s blood and compares them to a standard for their age group. Counts higher than 4,000 lymphocytes per μl of blood indicate lymphocytosis in adults.

Following a lymphocytosis diagnosis, doctors must determine the underlying cause. They may perform a physical exam, take a medical history, and ask about symptoms. They may also need to perform additional blood tests, bone marrow biopsies, and other diagnostic tests to determine the cause of lymphocytosis.

The causes of lymphocytosis vary considerably, so people should speak with a doctor if they have any concerning symptoms that persist for an extended period.

Individuals will require a blood test to determine if they have lymphocytosis as there are no other diagnostic methods.

Lymphocytosis refers to abnormally high levels of lymphocytes, which are a type of white blood cell. The condition can cause swelling of the lymph nodes and an enlarged spleen. It can be a sign of another problem that causes additional symptoms.

Medical conditions that cause lymphocytosis include infections, stress, and white blood cell cancers. Each of these conditions has its own symptoms and treatment.

Doctors diagnose lymphocytosis through a blood test. However, they may also use additional tests to determine the cause of lymphocytosis and decide on a suitable treatment plan.