Lymphocytes

Abstract

Lymphocytes are white blood cells that are derived from a common lymphoid progenitor in the bone marrow. They function as specific mediators involved in inducible immune responses against infectious agents. The classically defined adaptive B and T lymphocytes use defined surface molecules, antigen receptors, to detect antigens. The activation of lymphocytes requires multiple signals, including binding of the antigen‐specific receptor to the antigen, costimulation and/or exposure to environmental molecules. Activation results in an expansion of clonally reactive cells, which become effector cells (plasma cells and activated T cells) that function to eliminate pathogens, or which become memory cells to facilitate later secondary responses. A secondary group of lymphocytes, the innate lymphocytes (natural killer (NK) cells, γδ T cells, invariant NK T cells and B‐1 cells) also play a role in immune function. Specifically, they represent a pool of cells that respond quickly to a limited repertoire of antigens at sites of infection.

Key Concepts:

  • Cells of the lymphoid lineage are derived from the common lymphoid progenitors that differentiate from multipotent haematopoietic stem cells in the bone marrow.

  • The role of lymphocytes is to facilitate recognition of foreign antigens (immune stimulators) and to mount responses for their neutralization and/or elimination.

  • The developmental stages of B and T cells correspond to gene rearrangement events, leading to the different specificity of B cell and T cell receptors. B cells mature in the bone marrow, whereas T cells undergo education and maturation in the thymus.

  • B lymphocytes utilise an immunoglobulin receptor to recognise conformational antigens. Upon activation, they mature into plasma cells to secrete specific antibodies with directed target specificity.

  • T lymphocytes recognised processed antigen presented via major histocompatibility molecules. Multiple phenotypic subsets exist, allowing for differential regulation of cellular immune function.

  • T‐helper cells express CD4 molecules, and assist in both cellular and humoral functions. Specific helper activity is directed via secretion of pleiotropic cytokines.

  • Cytotoxic T cells express CD8 molecules, and target viral infected or tumour targets for destruction.

  • All T lymphocytes bear the CD3 molecule to assist in signalling once antigen is encountered.

  • The primary response of lymphocytes leads to the generation of ‘daughter clones’, which become effectors, a small fraction of which become memory cells. This generates a persistent population that can rapidly expand and respond with higher magnitude upon secondary encounter with an antigen.

  • The innate lymphocytes (natural killer cells, γδ T cells, invariant natural killer T cells and B‐1 cells) represent a special reservoir of cells that are able to respond quickly to a limited subset of antigens at sites of infection.

Keywords: lymphocytes; immune responses; T cells; B cells; natural killer T cells; adaptive immunity; helper T cells; cytotoxic T cells; antibodies

Figure 1.

Antigen receptors expressed as transmembrane molecules on B and T lymphocytes. Adapted from Coico and Sunshine ().

Figure 2.

(a) Structures of the major classes of secreted immunoglobulins. (b) Schematic representation of the immunoglobulin molecule (IgG) showing heavy and light chains, and relative domains. Adapted from Coico and Sunshine ().

Figure 3.

The TCRs interact with MHC molecules expressed on host presenting cells. (a) The CD4 lymphocytes interact with MHC class II expressed on profession presenting cells. (b) Whereas the CD8 lymphocytes interact with MHC class I which is expressed on all nucleated cells. Full activation is supported through additional interactions with both cosignalling and adhesions molecules, as well as cytokine activators. Adapted from Coico and Sunshine ().

Figure 4.

Gene rearrangements and subsequent steps in the synthesis of human heavy chains showing how alternative splicing of a B cell generates µ and δ chains of identical antigenic specificity. During development, recombination leads to the deletion of intervening stretches of DNA, generating a complete V‐region gene, consisting of fused VDJ segments in the case of the H chain, and VJ segments in the case of the L chain. Slicing to the C‐region genes precedes transcription into the complete polypeptide chains. A similar mechanism occurs for the generation of the T cell receptor. Adapted from Coico and Sunshine ().

Figure 5.

The major subsets of CD4+ T cells, with their signature cytokines and major effector functions indicated. Adapted from Coico and Sunshine ().

Figure 6.

Mechanisms of CD8 T cell function for target cell killing. Adapted from Coico and Sunshine ().

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Further Reading

Abbas AK and Lichtman AH (2011) Basic Immunology – Functions and Disorders of the Immune System, 3rd edn. Philadelphia, PA: Saunders.

Actor JK (2012) Integrated Immunology and Microbiology, 2nd edn. Philadelphia, PA: Mosby/Elsevier.

Kindt TJ, Goldsby RA and Osborne BA (2007) Kuby Immunology, 6th edn. New York: W.H. Freeman and Company.

Male D, Brostoff J, Roth DR and Roitt IM (2013) Immunology, 8th edn. China: Elsevier Ltd.

Murphy K (2012) Janeway's Immunobiology, 8th edn. New York, Philadelphia: Garland Publishing.

Parham P (2009) The Immune System, 3rd edn. New York: Garland Publishing.

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Hwang, Shen‐An, and Actor, Jeffrey K(May 2014) Lymphocytes. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001190.pub2]