Thymus lymphocytes or T cells are a specific type of white blood cells or leukocytes developed in the thymus organ that play a central role in the immune response and the adaptive immune system. Specifically, T cells attack cells infected by a pathogen, activate other immune cells, and produce cytokines and chemokines, while also playing a role in regulating the entire immune response.
Immune Response: The Roles of Thymus Lymphocytes in the Immune System
Leukocytes are the cells of the immune system responsible for protecting the body against infectious diseases and foreign invaders such as viruses and bacteria, as well as for responding to injuries caused by stress or trauma. They are also part of the mechanism for monitoring and eliminating the proliferation of cancerous cells and the growth of tumors.
On the other hand, as one of the five major types of leukocytes, the primary role of T cells center on protecting the body against pathogens and cancer cells. They represent the cell-mediated component of the adaptive immune system. Note that B lymphocytes or B cells represent the humoral component of adaptive immunity.
It is also important to highlight the fact that cell-mediated immunity works through the activation of immune cells to confront infections, foreign invasion, and cancer growth. This is the principal domain of thymus lymphocytes. Meanwhile, humoral immunity via the actions of the B cells primarily involves the secretion of antibodies.
T cells also differ from another type of leukocytes called macrophages. Macrophages engulf and digest cellular debris and cancer cells, as well as any pathogen and foreign substance. On the other hand, T cells usually concentrate on responding to an infection or abnormality caused by a specific type of pathogen or cancer cell.
Nevertheless, the specific functions of thymus lymphocytes depend on their particular type. Some of them are involved in direct immune response while others contribute to the regulation of the immune system. Most of these cells are part of the adaptive immunity, but some function as a bridge between adaptive and innate immune response.
Production, Activation, and Differentiation: The Major Types of T Cells
T cells originate from hematopoietic stem cells in the bone marrow as precursor cells, and they migrate to the thymus to develop further. They specifically mature and differentiate into distinct types of T cells while in the organ before migrating to the periphery. Note that differentiation continues even after leaving the thymus.
Understanding further the roles of T cells require understanding their distinctive types and their specific functions in the immune system. There are two main categories groping the different types of thymus lymphocytes. These are conventional adaptive T cells and innate-like T cells.
The conventional adaptive category constitutes four known types: T helper cells or CD4+ cells, cytotoxic T cells or CD8+ cells, memory T cells, and regulatory or suppressor T cells. Take note of the following specific functions:
• T helper cells or CD4+ cells: Responsible for releasing T cell cytokines needed for immune response signaling, as well as for activating other lymphocytes or white blood cells, especially by assisting in the maturation of B cells into plasma cells and memory B cells, stimulating B cells to produce and secrete antibodies, and assisting in activating cytotoxic T cells and macrophages. T helper cells do not directly kill infected cells, pathogens, and malignant cells.
• Cytotoxic T cells or CD8+ cells: Destroy virus-infected cells, tumor cells, and damaged cells upon signals from CD4+ cells and dendritic cells, among others. Most of these cells express T-cell receptors that recognize a specific antigen released by viruses, cancerous cells, or damaged cells. They work by releasing cytotoxic granules inside the targeted cells. Cytotoxins are substance toxic to cells. Note that they can also kill cells with traces of intracellular bacteria.
• Memory T cells: Responsible for assisting the immune system recognize previously recognized antigens from a previous infection or internal malignancy so that the immune response would be quicker and more efficient. They essentially have some of the same functions as memory B cells. Note that CD4+ cells and CD8+ cells could differentiate to become memory T cells. However, their exact origins or the reason behind their maturation remain debatable.
• Regulatory CD4+ cells: Controls the activity of T cells primarily by suppressing their response. They specifically shut down immunity mediated by thymus lymphocytes toward the end of an immune reaction and suppress autoreactive thymus lymphocytes. They are essential for modulating the immune system, maintaining tolerance to self-antigens, and preventing autoimmune diseases and chronic inflammation. They have four basic suppression mechanisms: suppression by inhibitory cytokines, suppression by cytolysis, suppression by metabolic disruption, and suppression by modulation of dendritic-cell maturation or function.
Aside from the above conventional adaptive thymus lymphocytes, there are three other types of T cells under the innate-like category. These are natural killer T cells, mucosal-associated invariant T cells, and gamma delta T cells. Take note of their following functions:
• Natural killer T cells: They share the properties of T cells and natural killer cells. They should not be confused with another type of lymphocytes called natural killer cells. Nevertheless, their functions include production of cytokines, release of cytotoxins, and recognition and elimination of some virus-infected cells, tumor cells, damaged or abnormal cells. A specific subtype known as invariant NKT regulates inflammation in adipose tissue, thus playing a role in protecting the body against obesity.
• Mucosal associated invariant T cells: Demonstrate innate and effector-like or helper-like characteristics. Specifically, they are abundant in tissues and areas exposed to microbial antigens such as the gut, liver, blood, lungs, and the mucosa. They have antimicrobial activities and work in tackling infections.
• Gamma delta T cells: They are differentiated from CD4+ cells and CD8+ cells that express alpha beta T cell receptors. Instead, they feature gamma and delta chains on their cellular surface. They can directly attack target cells through their cytotoxic activity or indirectly through the activation of other immune cells via cytokine release. Further, they have the ability to recognize a broad range of antigens without the presence of major histocompatibility complex molecules.
FURTHER READINGS AND REFERENCES
- Doherty, P. C. 1980. “Cytotoxic T Cells.” In Strategies of Immune Regulation. Elsevier. DOI: 1016/B978-0-12-637140-6.50038-3
- Kumar, B. V., Connors, T., and Farber, D. L. 2018. “Human T Cell Development, Localization, and Function Throughout Life.” Immunity. 48(2): DOI: 1016/j.immuni.2018.01.007
- Lawand, M., Déchanet-Merville, J., and Dieu-Nosjean, M-C. 2017. “Key Features of Gamma-Delta T-Cell Subsets in Human Diseases and Their Immunotherapeutic Implications.” Frontiers in Immunology. 8. DOI: 3389/fimmu.2017.00761
- Omilusik, K. D., and Goldrath, A. W. 2017. “The Origins of Memory T cells.” Nature. 552(7685): 337–339. 1038/d41586-017-08280-8
- Vignali, D. A. A., Collison, L. W., and Workman, C. J. 2008. “How Regulatory T cells Work.” Nature Reviews Immunology. 8(7): 523–532. DOI: 1038/nri2343