T lymphocytes

T lymphocytes are responsible for the immune response of cellular type (Hanna et al. 2004) precursor T cells produced in the bone marrow and mature as T lymphocytes in the thymus (Kohler and Thiel 2009). During the maturation period, many specific surface molecules and receptors fit on the surface of T lymphocyte. There is no surface immunoglobulin on the T cell surface. Instead, there is a T cell receptor (TCR) that specifically recognizes antigens. One T lymphocyte carries a TCR for only one type of antigen (Ozato, Tsujimura, and Tamura 2002). Moreover, cluster of differentiation antigens (CDs), which are specific surface molecules on the T cell surface, are formed and the ability to respond to antigenic stimulation is obtained. One of the CDs, CD4+, shapes helper T cells and CD8+

shapes the cytotoxic T cells (Beyaz 2004). When an antigen enters the organism, it finds and stimulates T lymphocytes carrying the receptor, which is specific to the antigen, on the cell surface. Stimulated T lymphocytes transform and as a result, antigen-sensitive T lymphocytes are formed. T lymphocytes are the most important cells of the immune system and form a specific immunity which is not directly dependent on the antibody and is directed and joined by the cells (Mosmann and Coffman 1989).

T cells are mainly divided into four subgroups:

  • Cytotoxic T cells (CT): There are CD8+ cells on the surface of these cells (Seder, Darrah, and Roederer 2008). They fight cells which are capable of directly eliminating foreign cells entering the organism (Kaech, Wherry, and Ahmed 2002).
  • Helper T cells (TH): There are CD4+ cells on the surface of these cells (Seder, Darrah, and Roederer 2008). Helper T cells constitute most of the T cells. These cells activate other T cells and В cells. When the stimulation begins, these cells synthesize cytokines. The main cytokines released from helper T lymphocytes are interleukin-2 (IL-2), interleu- kin-3 (IL-3), interleukin-4 (IL-4), interleukin-5 (IL-5), interleukin-6 (IL-6), tumor necrosis factor (TNF-p), and interferon gamma (IFN-y) (Kaech, Wherry, and Ahmed 2002; Seder. Darrah, and Roederer 2008; Kohler and Thiel 2009). Helper T lymphocytes have two subsets of TH1 and TH2 (Parker 1993; Mosmann and Coffman 1989). While TH1 is effective in the humoral immune response, TH2 is effective in the cellular immune response (Seder, Darrah, and Roederer 2008; Beyaz 2004; Bird et al. 1998). TH1 lymphocytes produce IFN-y and IL-2. TH2 lymphocytes produce IL-4, IL-5, and IL-10 (Yurdakok and ince 2008; Bird et al. 1998)
  • Suppressor T cells (TS): These cells have CD8+, similar to CT. They inhibit the activation of T and В cells (Beyaz 2004; Maughan, Preston, and Williams 2015).
  • Memory T Cells: These cells have CD4+, similar to TH (Hanna et al. 2004; Kohler and Thiel 2009; Kaech, Wherry, and Ahmed 2002). Storing the first antigenic stimulus, they detect the antigen and provide an immune response when they are stimulated with the same antigen for the second time (Beyaz 2004).
  • • Monocytes, Macrophages, and Dendritic Cells

Monocytes and tissue macrophages belong to the mononuclear phagocytic system, which is a part of the reticuloendothelial system. All cells of the mononuclear phagocytic system consist of common precursors in the bone marrow which produces blood monocytes (Elgert 2009b). Monocytes are transferred to various tissues and mature as macrophages in those tissues. Tissue macrophages are found in connective tissues in scattered form or in organs and other areas in the form of clusters. Macrophages have an important role in both non-specific and antigen- specific immune responses (Defranco, Locksley, and Robertson 2007). As phagocytic cells, they contribute to immunity by keeping infected agents in their body until a specific immunity develops (Hanna et al. 2004). Moreover, it functions in the early stage of the host response, during the enhancement of the inflammatory response, and the onset of a specific immunity. Dendritic cells share an important role with macrophages such as presenting antigens to T lymphocytes (Hanna et al. 2004; Vallhov et al. 2012; Maughan, Preston, and Williams 2015). Dendritic cells are found in lymphoid tissues and the areas where the body comes into contact with an antigen (Defranco, Locksley, and Robertson 2007).

Natural Killer Cells (NKs)

Among the lymphoid cells, they are the cells which eliminate infected or foreign cells, with a different structure than T and В lymphocytes (Hanna et al. 2004). Natural killer cells (NKs) are modulators of innate and adaptive immune responses (Tang et al. 2020).

NKs have the ability to directly destroy the cells they target before they recognize and become sensitive to them (Elgert 2009b).

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