Fish Immunology (1+1)

Unit 7- Lymphocytes

7.4. The life history of T lymphocytes

There are several subsets of T cells, each with a distinct function they are :-

• Cytotoxic T cells (CD8)
• T helper cells (CD4).
• Regulatory T Cells (Treg cells)

T helper cells (CD4):
T helper cells are also known as CD4+ T cells because they express the CD4 protein on their surface. Helper T cells are activated by the antigen presenting cells (APCs). Activated T cell secretes small proteins called cytokines that assist in the active immune response include maturation of B cells into plasma cells and memory B cells, and activation of cytotoxic T cells and macrophages.
Cytotoxic T cells:
Cytotoxic T cells are called "killer cells" because they produce chemical substances known as lymphokines. These cells are also known as CD8+ T cells because they express the CD8 on their surface. These cells destroy virally infected cells and also tumor cells. These cells recognize their targets by binding to antigen associated with MHC class I, which is present on the surface of nearly every cell of the body.
Regulatory T Cells (Treg cells):
Treg cells formerly known as suppressor T cells can regulate the mechanisms to prevent sustained inflammatory responses that may attacks on healthy tissue. Regulatory T cells are involved in preventing autoimmunity, moderating inflammation, and minimizing tissue damage. The function of regulatory T cells is to suppress the function of antigen-presenting cells and effector T cells (Teff cells). If Treg cell activity is reduced it may cause autoimmune diseases.
Treg cells are divided into 3 subsets.

• Naturally occurring CD4+CD25+
• Tr1 cells and
• Th3 cells

Naturally occurring CD4+CD25+
Naturally occurring CD4+CD25+ Treg cells develops in the thymus and performs the function like Induction of cytolysis, Disruption of metabolic activities, Inhibition of dendritic cell maturation and Secretion of IL-10, IL-35, TGF-b1, Galectin-1. This cells plays a major role in immune control.
Tr1 cells
Develop in the periphery during a normal immune response. Perform the function like cell-cell contact, secretion of IL-10, TGF-b1 and IFN-g
Th3 cells
Develop in the periphery during a normal immune response and secret TGF-b1.