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A lymphocyte is a type of white blood cell (leukocyte) in the immune system of gnathostomata. Lymphocytes include (for cell-mediated and cytotoxic adaptive immunity), (for humoral immunity, antibody-driven adaptive immunity), (2025). 9780323530453, Elsevier. ISBN 9780323530453 (2025). 9780323085939 ISBN 9780323085939 and innate lymphoid cells (ILCs; "innate T cell-like" cells involved in mucosal immunity and homeostasis), of which natural killer cells are an important subtype (which functions in cell-mediated, cytotoxicity innate immunity). They are the main type of cell found in lymph, which prompted the name "lymphocyte" (with cyte meaning cell). Lymphocytes make up between 18% and 42% of circulating white blood cells.
They can also be classified as small lymphocytes and large lymphocytes based on their size and appearance. (2025). 9780723436911 ISBN 9780723436911
Lymphocytes can be identified by their large nucleus.
The lymphocytes involved in adaptive immunity (i.e. B and T cells) differentiate further after exposure to an antigen; they form effector and memory lymphocytes. Effector lymphocytes function to eliminate the antigen, either by releasing antibodies (in the case of B cells), cytotoxic granules (cytotoxic T cells) or by signaling to other cells of the immune system (helper T cells). Memory T cells remain in the peripheral tissues and circulation for an extended time ready to respond to the same antigen upon future exposure; they live weeks to several years, which is very long compared to other leukocytes.
It is impossible to distinguish between T cells and B cells in a peripheral blood smear. Normally, flow cytometry testing is used for specific lymphocyte population counts. This can be used to determine the percentage of lymphocytes that contain a particular combination of specific cell surface proteins, such as or cluster of differentiation (CD) markers or that produce particular proteins (for example, using intracellular cytokine staining (ICCS)). In order to study the function of a lymphocyte by virtue of the proteins it generates, other scientific techniques like the ELISPOT or secretion assay techniques can be used.
In the circulatory system, they move from lymph node to lymph node.
A general increase in the number of lymphocytes is known as lymphocytosis, whereas a decrease is known as lymphocytopenia.
One basis for low T cell lymphocytes occurs when the human immunodeficiency virus (HIV) infects and destroys T cells (specifically, the CD4+ subgroup of T lymphocytes, which become helper T cells).
> + Typical recognition markers for lymphocytes
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(2025). 9780128175729 ISBN 9780128175729 This contrasts with macrophages, which are rather stationary in the nodes.
Lymphocytes and disease
A lymphocyte count is usually part of a peripheral complete blood cell count and is expressed as the percentage of lymphocytes to the total number of white blood cells counted.
High
An increase in lymphocyte concentration is usually a sign of a viral infection (in some rare cases, are found through an abnormally raised lymphocyte count in an otherwise normal person). (2025). 9781437702712 ISBN 9781437702712 A high lymphocyte count with a low neutrophil count might be caused by lymphoma. Pertussis toxin (PTx) of Bordetella pertussis, formerly known as lymphocytosis-promoting factor, causes a decrease in the entry of lymphocytes into lymph nodes, which can lead to a condition known as lymphocytosis, with a complete lymphocyte count of over 4000 per microlitre in adults or over 8000 per μl in children. This is unique in that many bacterial infections illustrate neutrophil-predominance instead.
Lymphoproliferative disorders
Lymphoproliferative disorders (LPD) encompass a diverse group of diseases marked by uncontrolled lymphocyte production, leading to issues like lymphocytosis, lymphadenopathy, and bone marrow infiltration. These disorders are common in immunocompromised individuals and involve abnormal proliferation of T and B cells, often resulting in immunodeficiency and immune system dysfunction. Various gene mutations, both iatrogenic and acquired, are implicated in LPD. One subtype, X-linked LPD, is linked to mutations in the X chromosome, predisposing individuals to natural killer cell LPD and T-cell LPD. Additionally, conditions like common variable immunodeficiency (CVID), severe combined immunodeficiency (SCID), and certain viral infections elevate the risk of LPD. Treatment methods, such as immunosuppressive drugs and tissue transplantation, can also increase susceptibility. LPDs encompass a wide array of disorders involving B-cell (e.g., chronic lymphocytic leukemia) and T-cell (e.g., Sezary syndrome) abnormalities, each presenting distinct challenges in diagnosis and management.
Low
A low normal to low absolute lymphocyte concentration is associated with increased rates of infection after surgery or trauma. (2025). 9780323045797 ISBN 9780323045797
(2025). 9780128149645 ISBN 9780128149645 Without the key defense that these T cells provide, the body becomes susceptible to opportunistic infections that otherwise would not affect healthy people. The extent of HIV progression is typically determined by measuring the percentage of CD4+ T cells in the patient's blood – HIV ultimately progresses to acquired immune deficiency syndrome (AIDS). The effects of other or lymphocyte disorders can also often be estimated by counting the numbers of lymphocytes present in the blood.
Tumor-infiltrating lymphocytes
In some cancers, such as melanoma and colorectal cancer, lymphocytes can migrate into and attack the tumor. This can sometimes lead to regression of the primary tumor.
Lymphocyte-variant hypereosinophilia
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