T- cells are the mediators of cell mediated immune response. T cells originate in bone marrow and mature in thymus. T cells interaction with other immune cells is via a receptor called T-Cell receptor (TCR), which can only interact with antigenic peptides bound to Class II MHC molecules on the surface of antigen presenting cells (APCs).
Mode of action:
When a TCR of naïve T cell interacts with an antigen MHC class II complex, it gets activated and undergoes differentiation and proliferation refers as clonal selection.
The T cells differentiates into 2 types of cells
1) Effector T cells : There are two types of effector T cells
2) Memory T cells: has long life span and is responsible for immunologic memory.
Effector T cells:
a) T helper cells or TH cells which has CD4 surface antigen and b) T cytotoxic cells or TC cells which has CD8 surface antigen.
T helper cells interact only with antigen-MHC class II complex. Upon activation, it proliferates and secretes variety of growth factors cytokines. Cytokines in turn activates T cells and B cells. The intensity of immune response depends on the production of cytokinins.
T cytotoxic cells or Tc cells recognizes antigen-MHC class I complex and proliferates under the influence of interleukin-2 (IL2) secreted by activated TH cells. The CD8 molecule of Tc cells associate with MHC class I molecule and activates Tc cells. These cells differentiate to form cytotoxic T lymphocytes (CTLs).
CTLs has cytotoxic activity and directly kills virus infected cells, tumour cells and and tissue graft cells. It has two enzymes, perforins and granyzyme that ensures the destruction of infected cells. Perforins create pores on the cell membrane and granyzymes are proteses that degrade cell contents.
Exogenous antigens or antigens produced outside host cells are generally phagocytised by APCs. They are degraded and antigenic peptides are displayed on the cell surface in association with Class II MHC molecules. These cells first interact with TH cells.
In the case of endogenous antigens, antigens produced within the host cells, like viral proteins, cancer cells surface proteins etc are degraded in the cytosol by proteasome complex. This peptide fragment binds to MHC class I molecules. These cells with Class I MHC complex with antigenic peptides activates TC cells to differentiate into CTLs.
The cytokines secreted by TH cells regulate the proliferation and differentiation of B cells and Tc cells and also a number of non-specific effector cells like natural killer cells (NK) cells and activated macrophages. Activities of these cells are non-specific and don’t have immunologic memory.For more: Immunology MCQs/Immunology notes