T- Lymphocytes is a major class of lymphocytes whose precursors arise from the bone marrow then migrate and mature in the thymus of which they derive their name (Thymus derived lymphocytes)
They are divided into functionally distinct populations which are T- helper cells (CD4+), cytotoxic cells (CD8+) and the T-regulatory cells (CD4+). T helper cells differentiate to the favored immune response in that if the most relevant immune response is by antibodies, it differentiates to help antibody immune response, and if it’s by cell cytotoxicity or phagocytosis, it differentiates to support that. T reg, On the other hand, regulates the proliferation of T helper and cytotoxic T cells.
The main function of the T cells is to regulate the immune responses to protein antigens and to serve as effector cells for the elimination of the intracellular microbes. T- Lymphocytes, therefore, do not produce antibodies; however, they have antigenic receptors that molecular distinct but related to the antibodies.
T helper cells and the cytotoxic T cells have an unusual specificity for antigens. They recognize only peptide antigens attached to proteins that are encoded by genes in the Major Histocompatibility Complex and expressed on the surface of other cells. In humans, MHCs are known as the Human Leukocyte Antigens. As a result, they respond to only the cell surface antigens and not soluble ones.
In response to antigenic stimulation, the T helper cells secrete protein hormones known as the cytokines. Which happens when they recognize the antigens presented to them together with the MHC class II molecules. Cytotoxic T cells, on the other hand, lyse cells that produce foreign antigens such as those infected by viruses and other intracellular microbes. They recognize antigens presented to them with the MHC class I molecules
The main aim of T helper cells is to secrete cytokines that promote other immune responses and are divided into T helper 1 which upon stimulation secret a group of cytokines that activate Cytotoxic T cells and T helper 2 which upon stimulation secret a group of cytokines that stimulate B-Cell to produce antibodies.