What Are Lymphocytes?

Medically Reviewed by Dan Brennan, MD on May 11, 2021

The color of your blood comes from your red blood cells. These carry oxygen through your body. You have fewer white blood cells than red blood cells. But they play an important role. They defend your body from infection.

Lymphocytes are a type of white blood cell. They’re an important part of your immune system. About 20% to 40% of your white blood cells are lymphocytes.

Function of Lymphocytes

For every 600 to 700 red blood cells, you have one white blood cell. They may be fewer, but they’re mighty. They act like your body’s army. They’re dispersed throughout your body, but once an organism or virus invades, they immediately gather to fight it off.

Lymphocytes are formed in your bone marrow. There are two types of lymphocytes:

  • B cells (B lymphocytes). These make antibodies. Antibodies can destroy foreign substances or tag them for attack.
  • T cells (T lymphocytes). These lymphocytes destroy any of your cells that have been taken over by viruses or cancers.

There is also another type of lymphocyte called natural killer cells. These natural killer cells are able to destroy tumor cells without any prior activation. This is unlike the T cells which need to be activated by another immune cell.

Lymphocytes are able to remember antigens, which are foreign substances that provoke your body’s immune reaction. These include bacteria, viruses, and toxins. After an encounter with an antigen, some lymphocytes develop into memory cells. When these memory lymphocytes meet an antigen for a second time, they respond rapidly and specifically to this antigen. This is why vaccines can prevent certain disorders.

How to Test for Lymphocytes

There are a few blood tests that test for white blood cells. 

Complete blood count (CBC). A complete blood count is a blood test that measures different components of your blood. It measures:

  • Red blood cells
  • White blood cells
  • Platelets, the cells which help with blood clotting
  • Hemoglobin, the protein in your blood that carries oxygen
  • Hematocrit, which is the ratio of red blood cells to plasma, the fluid component of your blood

If your blood is only to be used for the complete blood count, you don’t need to fast and can eat and drink normally. A healthcare professional will take a sample of your blood and send it in to a lab for testing. 

Flow Cytometry. This is a commonly used test that looks at many types of blood cells. It’s more detailed than a complete blood count and can measure the levels of different lymphocytes.

Your healthcare professional will take a sample of your blood and send it to a lab. A technician will suspend your blood sample in a fluid and send it through an instrument called a flow cytometer.

This instrument contains lasers and detectors. As your blood cells pass through the lasers, they scatter light in certain patterns that are detected by the flow cytometer. This allows the number and types of cells in your sample to be counted. Thousands of cells can be analyzed in a few minutes.

Lymphocyte Counts

The normal ranges of lymphocyte counts may vary among different labs. Labs may have different ways of measuring blood test results or they may test different samples. Talk to your doctor about your blood test results and what the numbers mean.

Normal lymphocyte ranges depend on your age. For adults, normal lymphocyte count is between 1,000 and 4,800 lymphocytes per microliter of blood. For children, it’s between 3,000 and 9,500 lymphocytes per microliter of blood.

Causes of High Lymphocyte Count

Lymphocytosis is when you have a higher than normal number of lymphocytes. Having a high lymphocyte count means that your body is fighting an infection. It’s usually temporary and harmless. It often goes away when your underlying condition improves.

But sometimes a high lymphocyte count can be due to a serious condition. Some causes of lymphocytosis include:

Having significantly more than 3,000 lymphocytes in 1 microliter of blood is considered to be a high lymphocyte count in adults. For children, it can be as high as 9,000 lymphocytes per microliter. The ranges for lymphocytosis can vary slightly depending on the type of test.‌

Diagnosis of lymphocytosis. Your doctor will talk to you about your medical history and symptoms, then do a physical exam. Besides blood tests like a complete blood count and flow cytometry, your doctor may also order a bone marrow biopsy. 

Causes of Low Lymphocyte Count

Lymphocytopenia or lymphopenia is when you don't have enough lymphocytes.

If your lymphocyte count is less than 1,000 lymphocytes per microliter of blood, this is generally considered to be lymphocytopenia in adults. In children, a low lymphocyte count is when it's less than 3,000 lymphocytes per microliter of blood. The normal ranges of lymphocytes can vary depending on the test.‌

Some underlying conditions may cause low T lymphocyte numbers, while others may cause low B lymphocyte numbers.

A number of conditions can cause low lymphocyte count:

There are also some very rare inherited conditions that cause lymphocytopenia. These are:

Diagnosis of lymphocytopenia. To test for underlying conditions causing low lymphocyte count, your doctor may order blood tests, lymph node tests, and bone marrow tests. In order to test a lymph node, it may need to be surgically removed. 

Show Sources


British Society for Immunology: “Natural Killer Cells.”

Cleveland Clinic: “Lymphocytosis.”

Introduction to Flow Cytometry: A Learning Guide. Becton, Dickinson, and Co.; 2002.

Mayo Clinic: “Complete blood count (CBC),” “Lymphocytosis.”

Merck Manual: “Acquired Immunity,” “Components of Blood.”

National Institutes of Health (NIH): “Lymphocytopenia.”

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