Lymphocytes are a type of white blood cell. These cells are created by your bone marrow. B cell lymphocytes stay in your bone marrow, while T cell lymphocytes travel to your thymus.

White blood cells are an important part of your immune system. They help your body fight antigens, which are bacteria, viruses, and other toxins that make you sick. If your doctor says you have a weakened immune system, that means there aren’t enough white blood cells in your bloodstream (1).

Your bone marrow constantly produces cells that will become lymphocytes. Some will enter your bloodstream, but most will move through your lymphatic system. The lymphatic system is the group of tissues and organs, like the spleen, tonsils, and lymph nodes, that protect your body from infection (1).

About 25 percent of the new lymphocytes remain in the bone marrow and become B cells. The other 75 percent travel to your thymus and become T cells (2).

There are different kinds of B cells and T cells. These include:

  • effector cells that are activated by antigens to fight an active infection
  • memory cells that have been in your body long enough to recognize and “remember” past infections and go into action quickly if you become re-infected with an antigen

B lymphocytes and T lymphocytes work together to fight infection.

B lymphocytes recognize antigens and become plasma cells that produce antibodies to fight them.

There are three types of T lymphocytes, and each plays its own role. These include:

  • cytotoxic T cells
  • helper T cells
  • regulatory T cells

Cytotoxic T cells, often called killer T cells, destroy cells in your body that have been infected with an antigen, cancer cells, and foreign cells like transplanted organs. Helper T cells direct the immune response of B cells and other T cells (2).

Regulatory T cells suppress your immune system to keep its response in check. In addition to preventing autoimmune disease, they also prevent other white blood cells from fighting real or perceived antigens. Perceived antigens include substances like allergens and normal flora bacteria in the gastrointestinal tract. Allergens are things that cause an allergic reaction, which could include pollen, molds, or pet dander (1, 2).

If you show symptoms of an infection and suspected blood disorder your overall lymphocyte count is abnormal. If this is the case, your doctor may order a blood test called a B and T cell screen to count how many lymphocytes are in your bloodstream. Lymphocyte counts that are too high or too low can be a sign of illness.

For the test, blood is drawn from your arm at your doctor’s office or a lab. The blood is then sent to a laboratory where it will be analyzed. The results will be sent to your doctor. Ask your doctor when you should expect the results. Depending on the lab, wait times can vary, but typically take no more than two to three days.

There are a few things that can affect the results of your blood test. Tell your doctor if any of the following apply to you:

Laboratories often have different ways of measuring blood test results. Results also vary by:

  • age
  • gender
  • heritage
  • how high above sea level you live

The following table gives approximate ranges for adults, but you’ll want to talk to your doctor about your results. Some factors could determine what’s a normal range for you.

TestAdult normal cell countAdult normal range (differential)Low levelsHigh levels
white blood cells (WBC)4,500-10,000 (4.5-10.0) white blood cells/mcL1% of total blood volumecritical when fewer than 2,500 lymphocytes/mcLcritical when greater than 30,000/mcL
lymphocytes800-5000 (0.8-5.0) lymphocytes/mcL18-45% of total white blood cellsfewer than 800 lymphocytes/mcLgreater than 5,000 lymphocytes/mcL

A low lymphocyte count, called lymphocytopenia, usually occurs because:

  • your body isn’t producing enough lymphocytes
  • lymphocytes are being destroyed
  • lymphocytes are trapped in your spleen or lymph nodes

Lymphocytopenia can point to a number of conditions and diseases. Some, like the flu or mild infections, aren’t serious for most people. But a low lymphocyte count puts you at greater risk of infection.

Other conditions that can cause lymphocytopenia include:

Lymphocytosis, or a high lymphocyte count, is common if you’ve had an infection. High lymphocyte levels that persist may point to a more serious illness or disease, such as:

You may find it helpful to ask the following questions if your doctor orders a B and T cell screen:

  • Why do you think I need this test?
  • Are you testing for a specific condition?
  • Do I need to do anything special to prepare for the test?
  • How soon will I get the results?
  • Who will give me the results and explain them to me?
  • If the test results are normal, what will the next steps be?
  • If the test results are abnormal, what will the next steps be?
  • What self-care steps should I be taking while waiting for the results?

Lymphocyte counts that are either too low or too high may mean you have an infection or mild illness. When you recover, your lymphocyte levels will return to normal. If abnormal lymphocyte counts persist, your outlook depends on the underlying condition.