What Are Lymphocytes?

Understand Their Significance

Certain white blood cells are classified as lymphocytes. Types of lymphocytes include B cells, T cells, and NK cells. They are involved in the immune system’s memory to give you long-lasting immunity against viruses, bacteria, and other pathogens (infectious organisms) you have been exposed to.

Your approximate number of lymphocytes can be measured with a blood test. High levels or low levels can be temporary responses to minor health issues, like a mild infection or a medication side effect.

Prolonged or major changes in the number of lymphocytes can indicate serious medical problems, such as cancer or AIDS (acquired immune deficiency syndrome).

A stained blood smear seen under the microscope with a lymphocyte in the center surrounded by red blood cells and a couple of platelets to the right

Ed Reschke / Getty Images

Lymphocyte Types and Function 

The two main categories of lymphocytes are B cells and T cells. B cells make antibodies, which are proteins that bind to antigens (molecules that may be present on invading organisms, foreign substances, or cancer cells) so that they can be destroyed.

Many of the body’s T cells release material to destroy harmful pathogens (organisms like bacteria, viruses, or fungi that cause disease).

NK cells (also called natural killer cells) are a type of lymphocyte that can release chemicals to kill tumor cells or cells containing viruses.

There are several subcategories of T cells:

  • Cytotoxic T cells, also CD8 cells, release chemicals that destroy pathogens.
  • Helper T cells, also called CD4 cells, release chemical messengers to attract immune cells and proteins that can help in fighting infection.
  • Regulatory T cells suppress the immune response to prevent an overactive immune system.

There are also several types of B cells:

  • Memory B cells are stored in the lymph nodes and throughout the body after they have already created antibody specificity to a past infection.
  • Plasma cells (a type of B cell) release antibodies during an active infection.

Some types of antibodies produced by B cells include the following immunoglobulin (Ig) antibiodies:

  • IgG antibodies: The most prevalent
  • IgM antibodies: Usually the first to respond to an infection
  • IgE antibodies: Involved in allergic reactions
  • IgA antibodies: Present in the body’s linings (such as the nasal cavity)
  • IgD antibodies: Play some role in allergic reactions and participate in respiratory immune defense

Lymphocytes and antibodies work together, along with other white blood cells that are not lymphocytes.

Lymphocytes are involved in ongoing immunity. After exposure to certain pathogens, some B cells, T cells, and antibodies will reactivate upon reexposure to the same pathogen at a later time, resulting in a faster and more effective immune response from the body. This memory immunity is activated by infections and by vaccination.

Lymphocyte Production and Maturation

Blood cells, including lymphocytes, are produced in the bone marrow. Immature B cells mature in the bone marrow. Immature T cells mature in the thymus, an organ in the neck.

Normally, some T cells and B cells circulate in the blood, and some circulate in the lymphatic vessels, a group of vessels that extends throughout the body and communicates with blood circulation.

When these immune cells are needed to fight an infection, they increase in number as the body releases more lymphocytes, and they can often be found in larger than normal numbers at the site of infection and in the circulation.

How Lymphocytes Are Measured

Your approximate number of lymphocytes can be estimated by a complete blood count (CBC) with differential that provides the concentration of each type of blood cell in a sample and identifies the different types of white blood cells present.

For certain conditions, specialized lymphocyte testing can be done. To monitor human immunodeficiency virus (HIV) treatment response, CD4 blood counts are measured.

A bone marrow biopsy may evaluate the number of developing lymphocytes, as well as any abnormalities in their structure—which can be present with cancer.

Lymphocytes and Cancer 

The number and function of lymphocytes can be affected in some types of hematological (blood cell) cancers, such as leukemias. A decrease in lymphocyte number and function can also be secondary to the use of cancer treatments like chemotherapy.

Types of changes in lymphocytes associated with cancer include:

  • Blood cell cancers are the type of cancer most commonly associated with a high lymphocyte count.
  • Solid tumor cancers (such as cancer of an organ, like the lungs or the liver) can be associated with a high number of lymphocytes at some stages of the illness.
  • Cancer may cause inflammation that increases lymphocytes.
  • Cancer may interfere with the production of healthy lymphocytes, leading to a low number of lymphocytes.
  • Many types of cancer treatments cause a temporary decrease in lymphocytes. Often after the treatment is completed, the lymphocyte number will recover.
  • Sometimes cancer treatment may cause lymphocytes to increase due to inflammation, but this response is less common than an increase.

Some of the blood cell cancers that are commonly associated with a high number of lymphocytes include:

The diagnosis of any of these types of blood cancer is based on the number of lymphocytes, as well as their microscopic appearance. Typically, when someone has blood cancer, the body produces an excess number of one or more types of white blood cells that do not function normally. There's not a specific level of lymphocytes that always indicates cancer.

Along with excessive white blood cell production, there may also be a deficiency in other white blood cells, red blood cells, and platelets (cells involved with blood clotting).

Blood Cancer Effects on Cell Counts

Blood cell cancers characteristically can cause some white blood cells to be elevated and others to be too low.

Causes of High Lymphocytes 

Typically, elevated lymphocytes are a sign of an infection. This can be seen in certain types of viral, bacterial, and fungal infections. This is generally a brief response to an infection as your body fights the pathogen so that you can recover.

Autoimmune diseases and allergic reactions may cause lymphocytes to increase occasionally.

Sometimes a high number of lymphocytes means there is a serious medical issue. One of the most concerning causes of an elevated number of lymphocytes is cancer.

Causes of Low Lymphocytes 

Low lymphocytes (lymphopenia) can be a sign of immune deficiency. Immune deficiency can be caused by medications or illnesses.

Medications associated with low lymphocytes include corticosteroids, immune suppressants like methotrexate, anti-inflammatories like Azulfidine (sulfasalazine), chemotherapy, and many more.

Conditions commonly associated with low lymphocytes include:

  • Vitamin deficiency
  • Malnutrition
  • AIDS
  • Bone marrow disease
  • Chronic disease

Additionally, autoimmune diseases can cause high or low lymphocytes. The treatment of many autoimmune conditions can cause low lymphocytes as well.

Lymphocytes as a Prognostic Indicator 

Sometimes the lymphocyte count can be a sign that a medical condition is likely to worsen or improve. For example, a high neutrophil-to-lymphocyte ratio is a poor prognostic factor in many advanced malignancies, and often the ratio is increased due to a decrease in the lymphocyte number.

Also, the lymphocyte distribution in the tumor (number and location) may also predict whether certain treatments are likely to be effective or not. For example, the location on the center or periphery of a tumor and the type of lymphocytic infiltration can be used to calculate an immune score that is an important prognostic factor in stage 2 colorectal cancer.

Summary 

Lymphocytes are a class of white blood cell. They include several types of T cells and B cells that make different kinds of antibodies. Lymphocytes are involved in the memory of your immune system, which means that they will give you lasting protection if you have a repeated exposure to an infectious organism.

There are many different causes of high or low lymphocytes. Usually, just one or a few types of lymphocytes are high or low, rather than all of them. Temporarily elevated lymphocytes can occur due to an infection or medication side effect.

In general, temporary variation in lymphocytes is not usually a cause for alarm. However, a lymphocyte number that increases or decreases over time can be caused by blood cell cancers, solid tumors, or immune deficiency.

Usually, blood cell cancer involves abnormal appearance of the lymphocytes rather than just a high number. The lymphocyte numbers can fluctuate with chronic conditions like recurrent allergies and autoimmune diseases.

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By Heidi Moawad, MD
Heidi Moawad is a neurologist and expert in the field of brain health and neurological disorders. Dr. Moawad regularly writes and edits health and career content for medical books and publications.