Lymphocyte

Cell Morphology

Lymphocytes are a type of white blood cell, or leukocyte, and are the chief cells of the immune system. There are three main classes of lymphocytes, B lymphocytes, T lymphocytes, and Natural Killer cells. Lymphocytes make 20–40% of the total leukocytes and 99% of cells in the lymphatic system (Owen et al., 2018). B lymphocytes make up 5–15% of human blood lymphocytes and T lymphocytes make up 70% of human blood lymphocytes (Todd, 2010). Natural killer cells make 10% of the lymphocyte population (Pawlina, 2016).

The majority of B and T lymphocytes are small. They have a round-shaped nucleus that occupies most of the cytoplasm. They also have little cytoplasm and condensed nuclear chromatin when inactive. Natural killer cells have a kidney-shaped nucleus and large cytoplasmic granules. The diameter of lymphocytes ranges from 5 to 17 μm. Activated lymphocytes are larger. Natural killer cells are larger than B and T lymphocytes (Pawlina, 2016).

B lymphocytes possess protein receptors for antigens and other proteins on their plasma membrane. T lymphocytes generally possess the CD4+ and CD8+ surface glycoproteins. Natural killer cells express the surface marker NK1 and cytotoxic granules (Owen et al., 2018).

T lymphocytes have a long lifespan while the lifespan of B lymphocytes varies. Natural killer cells live for around 2 weeks (Vivier et al., 2008).

Production

Lymphocytes originate in the red bone marrow from hematopoietic stem cells that give rise to lymphoid stem cells, which give rise to lymphocytes. However, lymphocytes mature in different sites. Development of lymphocytes takes days to weeks (Marieb, Wilhelm and Mallatt, 2012).

B lymphocytes complete their maturation in the bone marrow and T lymphocytes leave the bone marrow as immature cells and complete their maturation and development in the thymus. After their maturation, they enter the bloodstream and migrate to the lymphoid tissues.

Natural killer cells don't solely develop and mature in the bone marrow but in other lymphoid tissues, including the tonsils, spleen, and lymph nodes (Pawlina, 2016).

Function

Lymphocytes are responsible for fighting infectious agents by recognizing antigens, proteins, or glycoproteins located in the plasma membrane of a foreign cell and eliciting an immune response. B lymphocytes are one type of the antigen-presenting cells, meaning that they present the antigens to T lymphocytes for destruction. B lymphocytes differentiate into:

—plasma cells, which secrete antibodies (IgA, IgG, IgE, IgM, IgD) that bind to the antigens and mark them for phagocytosis;

—Memory B cells, that remain in the lymphoid tissues until the same antigen is encountered again and elicit an immune response more vigorously.

T lymphocytes differentiate into Helper T cells that assist with B lymphocyte differentiation via the secretion of cytokines, and Cytotoxic T cells, which kill the virus-infected host cells by causing them to lyse.

Natural killer cells recognize surface proteins of infected, cancerous, or stressed cells. When activated, natural killer cells kill such cells via apoptosis. Natural killer cells are important cells of the innate immunity as they aid to control an infection until an adaptive immunity is developed (Owen et al., 2018).

List of Clinical Correlates

—DiGeorge syndrome

—AIDS

—Mononucleosis (“kissing disease”)

—Hodgkin’s lymphoma

—Non-Hodgkin’s lymphoma

—Lymphocytosis

References

Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.

Marieb, E. N., Wilhelm, P. B. and Mallatt, J. (2012) Human Anatomy. 14th edn.: Benjamin Cummings.

Owen, J., Punt, J., Stranford, S. and Jones, P. (2018) Kuby Immunology. Macmillan Learning.

Pawlina, W. 2016. Histology: A text and atlas with correlated cell and molecular biology. 7th ed. Philadelphia: Wolters Kluwer.

Todd, I. (2010) 'Cells of the Immune System', eLS.

Vivier, E., Tomasello, E., Baratin, M., Walzer, T. and Ugolini, S. (2008) 'Functions of natural killer cells', Nat Immunol, 9(5), pp. 503-10.