T‐Lymphocyte Responses: Development


CD4+ T cells can be subdivided from a functional point of view into two main subsets: effector cells, which provide protection against exogenous agents, and regulatory T (Treg) cells whose function is to avoid autoimmune reactions and to stop the effector response against exogenous antigens, when the response itself becomes dangerous for the host.

Human effector T helper (Th) lymphocytes can be additionally classified into lineages based mainly on their immunological functions that are supported by distinct profile of cytokine, transcription factor and homing receptor expression. Recently, beyond the well‐known Th1 and Th2 cells, other populations have been discovered. These include the Th17 subset, which is certainly the most intensively studied, but also Th22, Th9 and T follicular helper (Tfh) lymphocytes. T effector lymphocytes are not only responsible for different types of protective responses but also for the pathogenesis of many disorders.

Regarding Treg cells, there are two main categories, one emerging from the thymus as a population already planned to suppress the response against self antigens and another population which also emerges from the thymus, but acquires its suppressive activity in the periphery and is devoted to regulating the response towards non‐self antigens.

Key Concepts

  • CD4+ Th cells exist in two main functional subsets: effector cells that fight with exogenous agents and regulatory cells that protect from the damage induced by immune response.
  • Effector Th lymphocytes can be distinguished into three main phenotypes based on their immunological functions, the Th1, Th2 and Th17 subsets.
  • Cytokines produced by cells of innate immunity at the moment of the encounter of the naive Th cell with the specific antigen are crucial in the acquisitions of the different effector phenotypes.
  • T effector lymphocytes are also responsible for the pathogenesis of many disorders, Th1 and Th17 cells playing a role in chronic inflammatory diseases and Th2 cells in atopic diseases.
  • T regulatory cells provide protection from autoimmune phenomena and from the potential damage induced by persisting of the immune response.
  • T regulatory cells that achieve the regulatory phenotype in the thymus protect from self‐reactions, while the ones that acquire the regulatory properties in the periphery suppress the responses to exogenous antigens, when they may become dangerous for the host.

Keywords: Th1; Th2; Th17; T regulatory cells; autoimmunity; allergy

Figure 1. Human effector T helper lymphocytes.
Figure 2. Natural occurring and adaptive Treg lymphocytes in humans.


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Cosmi, Lorenzo, Liotta, Francesco, and Annunziato, Francesco(Jun 2015) T‐Lymphocyte Responses: Development. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001244.pub3]