T cells (thymus cells) and B cells (bone marrow- orbursa-derived cells[a]) are the major cellular components of the adaptive immune response. T cells are involved incell-mediated immunity, whereas B cells are primarily responsible forhumoral immunity (relating toantibodies). The function of T cells and B cells is to recognize specific "non-self" antigens, during a process known asantigen presentation. Once they have identified an invader, the cells generate specific responses that are tailored maximally to eliminate specificpathogens or pathogen-infected cells. B cells respond to pathogens by producing large quantities ofantibodies which then neutralize foreign objects likebacteria andviruses. In response to pathogens some T cells, calledT helper cells, producecytokines that direct the immune response, while other T cells, calledcytotoxic T cells, produce toxicgranules that contain powerfulenzymes which induce the death of pathogen-infected cells. Following activation, B cells and T cells leave a lasting legacy of the antigens they have encountered, in the form ofmemory cells. Throughout the lifetime of an animal, these memory cells will "remember" each specific pathogen encountered, and are able to mount a strong and rapid response if the same pathogen is detected again; this is known as acquiredimmunity.
NK cells are a part of theinnate immune system and play a major role in defending the host fromtumors andvirally infected cells.[2] NK cells modulate the functions of other cells, includingmacrophages and T cells,[2] and distinguish infected cells and tumors from normal and uninfected cells by recognizing changes of a surface molecule calledmajor histocompatibility complex (MHC)class I. NK cells are activated in response to a family ofcytokines calledinterferons. Activated NK cells releasecytotoxic (cell-killing)granules which then destroy the altered cells.[1] They are named "natural killer cells" because they do not require prior activation in order to kill cells which are missing MHC class I.
The X lymphocyte is a reported cell type expressing both aB-cell receptor andT-cell receptor and is hypothesized to be implicated in type 1 diabetes.[8][9] Its existence as a cell type has been challenged by two studies.[10][11] However, the authors of original article pointed to the fact that the two studies have detected X cells by imaging microscopy and FACS as described.[12] Additional studies are required to determine the nature and properties of X cells (also called dual expressers).
Mammalianstem cellsdifferentiate into several kinds of blood cell within thebone marrow.[13] This process is calledhaematopoiesis.[14] All lymphocytes originate, during this process, from a common lymphoid progenitor before differentiating into their distinct lymphocyte types. The differentiation of lymphocytes follows various pathways in a hierarchical fashion as well as in a more plastic fashion. The formation of lymphocytes is known aslymphopoiesis. Inmammals, B cellsmature in thebone marrow, which is at the core of mostbones.[15] Inbirds, B cells mature in thebursa of Fabricius, a lymphoid organ where they were first discovered by Chang and Glick,[15] (B for bursa) and not from bone marrow as commonly believed. T cells migrate to the blood stream and mature in a distinct primary organ, called thethymus. Following maturation, the lymphocytes enter the circulation and peripherallymphoid organs (e.g. thespleen andlymph nodes) where they survey for invadingpathogens and/or tumor cells.
The lymphocytes involved in adaptive immunity (i.e. B and T cells) differentiate further after exposure to anantigen; 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.[citation needed]
It is impossible to distinguish between T cells and B cells in a peripheral blood smear.[13] 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 asimmunoglobulins orcluster of differentiation (CD) markers or that produce particular proteins (for example,cytokines 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 theELISPOT orsecretion assay techniques can be used.[1]
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Several lymphocytes seen collected around a tuberculousgranuloma
A lymphocyte count is usually part of a peripheralcomplete blood cell count and is expressed as the percentage of lymphocytes to the total number of white blood cells counted.
An increase in lymphocyte concentration is usually a sign of aviral infection (in some rare case,leukemias are found through an abnormally raised lymphocyte count in an otherwise normal person).[18][19] A high lymphocyte count with a lowneutrophil count might be caused bylymphoma.Pertussis toxin (PTx) ofBordetella 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 perμl in adults or over 8000 per μl in children. This is unique in that many bacterial infections illustrate neutrophil-predominance instead.
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.[20]
One basis for low T cell lymphocytes occurs when thehuman immunodeficiency virus (HIV) infects and destroys T cells (specifically, theCD4+ subgroup of T lymphocytes, which become helper T cells).[22] Without the key defense that these T cells provide, the body becomes susceptible toopportunistic 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 toacquired immune deficiency syndrome (AIDS). The effects of otherviruses or lymphocyte disorders can also often be estimated by counting the numbers of lymphocytes present in theblood.
In some cancers, such asmelanoma andcolorectal cancer, lymphocytes can migrate into and attack thetumor. This can sometimes lead to regression of the primary tumor.
^The process of B-cell maturation was elucidated in birds and the B most likely means "bursa-derived" referring to thebursa of Fabricius.[7] However, in humans (who do not have that organ), the bone marrow makes B cells, and the B can serve as a reminder ofbone marrow.
^abcdeOmman, Reeba A.; Kini, Ameet R. (2020). "Leukocyte development, kinetics, and functions". In Keohane, Elaine M.; Otto, Catherine N.; Walenga, Jeanine N. (eds.).Rodak's Hematology: Clinical Principles and Applications (6th ed.). St. Louis, Missouri: Elsevier. pp. 117–135.ISBN978-0-323-53045-3.
^"NCI Dictionary of Cancer Terms".National Cancer Institute. Retrieved22 July 2020.A type of immune cell that is made in the bone marrow and is found in the blood and in lymph tissue. The two main types of lymphocytes are B lymphocytes and T lymphocytes. B lymphocytes make antibodies, and T lymphocytes help kill tumor cells and help control immune responses. A lymphocyte is a type of white blood cell.
^Monga I, Kaur K, Dhanda S (March 2022). "Revisiting hematopoiesis: applications of the bulk and single-cell transcriptomics dissecting transcriptional heterogeneity in hematopoietic stem cells".Briefings in Functional Genomics.21 (3):159–176.doi:10.1093/bfgp/elac002.PMID35265979.
^Guilbert, Theresa W.; Gern, James E.; Lemanske, Robert F. (2010). "Infections and Asthma".Pediatric Allergy: Principles and Practice. Elsevier. pp. 363–376.doi:10.1016/b978-1-4377-0271-2.00035-3.ISBN978-1-4377-0271-2.S2CID78707217.Lymphocytes are recruited into the upper and lower airways during the early stages of a viral respiratory infection, and it is presumed that these cells help to limit the extent of infection and to clear virus-infected epithelial cells.
^Justiz Vaillant, Angel A.; Stang, Christopher M. (2023),"Lymphoproliferative Disorders",StatPearls, Treasure Island (FL): StatPearls Publishing,PMID30725847, retrieved7 October 2023
^Wahed, Amer; Quesada, Andres; Dasgupta, Amitava (2020). "Benign white blood cell and platelet disorders".Hematology and Coagulation. Elsevier. pp. 77–87.doi:10.1016/b978-0-12-814964-5.00005-x.ISBN978-0-12-814964-5.S2CID241056897.Lymphocytopenia may also be acquired, for example, in patients with HIV infection.
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