What is a lymphocyte?

The immune system is a complex network of cells and organs that work together to protect the body from foreign substances (antigens), such as bacteria, viruses, or atypical cells.

Lymphocytes are a subtype of white blood cell (leukocyte) that is a key element of the immune system. In adults, lymphocytes account for about 20-40% of the total number of white blood cells. Most lymphocytes are short-lived, with an average lifespan of a week to several months, but some live for years. Such long-lived cells constitute immunological memory. This means that if any antigen enters the body, lymphocytes will already know how to deal with it.

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Lymphocytes can be found in blood, lymph, peripheral lymphoid organs (lymph nodes, spleen) and tissues, where an initial immune response is usually formed.

Lymphocytes are formed in the bone marrow. About 75% of new lymphocytes from the bone marrow move to the thymus, where they become T cells. The remaining lymphocytes remain in the bone marrow and become B cells.

T- and B-lymphocytes got their names from the organs in which they develop. T cells develop in the thymus, and B cells in the bone marrow in adults. In the fetus, B cells develop largely in the liver. Despite their different origins, both T and B cells develop from the same stem cells, which give rise to all blood cells, including red blood cells, white blood cells, and platelets.


Lymphocytes have the ability to recognize a foreign protein (antigen) with which they once met. Their work is the body’s immune response to foreign invasion. After a lymphocyte binds to a foreign object (for example, a virus or a bacterium), it begins to divide, resulting in the formation of identical cells (clones). The ability to respond to almost any antigen is due to the huge diversity of populations. Due to this property of lymphocytes, your body develops immunity against various diseases.

There are different types of T-lymphocytes and B-lymphocytes, which perform various functions.

B cells are involved in the humoral immunity (or the antibody-mediated immunity (AMI)). They recognize foreign substances (antigens) and become plasma cells that produce antibodies.

In the thymus, T-lymphocytes are divided into the following cells:

  • Cytotoxic T cells (T-killers) destroy infected or malignant cancer cells.
  • Helper T cells (T-helpers) help other cells to trigger an immune response against foreign agents.
  • Regulatory T cells control or suppress other cells of the immune system.

Differential WBC Count‎

WBC differential reflects the relative (percentage) content of white blood cells of various types, and an increase or decrease in the percentage of lymphocytes may not reflect the true result but may be due to a decrease or increase in the absolute number of other types of WBCs (usually neutrophils). Therefore, it is always necessary to consider the absolute number of lymphocytes. The absolute content of lymphocytes can be determined by the formula:

LYM x WBC /100

  • LYM (%) – the percentage of lymphocytes
  • WBC (109/L) – total white blood cell count

Units of Measure

The absolute number of lymphocytes in the blood can be expressed in international units:

  • 109 cells/liter
  • G/L – Giga per Liter. (Giga = 1 billion)

Conventional units:

  • thousand per cubic millimeter (1000/mm3, 103/mm3)
  • thousand per microliter (1000/µL, 103/µL)
  • K/mm3 (thousand cells per cubic millimeter)
  • K/µL (thousand cells per microliter)
  • cells/mm3
  • cells/µL

Conversion factors:

Cubic millimeter (mm3) = microliter (mcL, µL)109 cells/Liter = G/L = 103/mm3 = 103/µL = K/mm3 = K/µL

Normal Range

Normal lymphocyte counts may vary by age, gender, and from lab to lab.

The normal range of lymphocytes in adults is from 1 to 4 x 109 cells/L (20-40%).

Normal Range in Pregnancy

During pregnancy, the reference values vary slightly:

  • First trimester: 1.1-3.6 x 109/L
  • Second trimester: 0.9-3.9 x 109/L
  • Third trimester: 1-3.6 x 109/L

Physiological pregnancy is characterized by certain changes in the cellular composition of the blood, which develop due to functional changes in the body of the expectant mother. Such changes are aimed to ensure a healthy pregnancy and baby.

During pregnancy, the body accelerates the metabolism, actively secretes hormones that stimulate cell division. The bone marrow, where white blood cells are formed, also increases its activity (WBC count increase by 20%). However, the number of lymphocytes in the blood during pregnancy remains unchanged or slightly decreases. As a rule, lymphocytes number in the blood remains acceptable (that is, it falls within the reference range).

Normal Range in Children

In children up to about 4 years of age, the number of lymphocytes exceeds the number of other types of white blood cells. At about 4 to 5 years, the level of lymphocytes and neutrophils becomes the same, after which the number of neutrophils begins to prevail.

The normal range of lymphocytes in children depends on age:

AgeReference Interval
2 weeks2.8-9.1
4 weeks 3.0-13.5
2-6 months4.0-10.5
6 months – 1 year4.0-10.5
1-6 years1.5-9.5
6-12 years1.5-7.0
12-18 years1.1-6.5

High Lymphocyte Count

An increased number of lymphocytes in the blood is called lymphocytosis.

A high level of lymphocytes may be an indication of many childhood diseases, primarily viral ones. An elevated level of lymphocytes is often seen during the recovery period after infectious diseases and intoxications – post-infection lymphocytosis. Lymphocytosis can be observed for another month after the illness, and sometimes longer.

A high level of lymphocytes can also be a sign of conditions:

  • acute viral infections (Influenza, hepatitis, mumps, measles, rubella, infectious mononucleosis, adenovirus, and many others)
  • autoimmune disorders causing chronic inflammation
  • cancer of the blood or lymphatic system
  • tuberculosis
  • syphilis
  • toxoplasmosis
  • whooping cough
  • cytomegalovirus (CMV)

Low Lymphocyte Count

A decrease in the number of lymphocytes in the blood is called lymphopenia (or lymphocytopenia).

A low number of lymphocytes may be temporary, resulting from intense physical exercise, severe stress, malnutrition, or after a cold or another infection.

Lymphopenia may occur in the following conditions:

  • autoimmune disorders (lupus, rheumatoid arthritis, myasthenia gravis)
  • prolonged infections (HIV, AIDS, tuberculosis)
  • cancer (leukemia or lymphoma)
  • blood diseases (aplastic anemia)
  • steroid use
  • radiation therapy and chemotherapy
  • some hereditary disorders (Wiskott-Aldrich syndrome and DiGeorge syndrome)


Lymphocytopenia is not a separate disease. You may not experience any symptoms, but in some cases, the symptoms of the underlying disease or condition appear:

  • fever
  • cough, runny nose
  • enlarged lymph nodes
  • weight loss
  • skin rash
  • painful joints

Complete Blood Count (CBC) Interpretation