The immune response which is mediated by the antigen specific T-cells (mainly cytotoxic and helper T-cell) of immune system and does not involve antibodies, is known as cell mediated immune response or cell mediated immunity. This immune response is associated with the secretion of cytokines.
- The helper T-cell, carrying CD4 marker, recognizes the antigen associated with class-II MHC molecule, and induces a specific immune response against this particular antigen by secreting some proteins (which acts as signalling molecule) like cytokines, lymphokines etc.
- The cytotoxic T-cell, carrying CD8 marker can recognize the antigen binding to the MHC-I molecule present on the surface of the host cells. It helps to destroy the intracellular target cell or the own infected cell of the host body as tumour cells, viral infected cells by lysis. It can also secrete the protein granzymes and perforin (it is also secreted by NK-cells), which can form pores on the target cells and induces lysis of the cells.
MECHANISM OF REACTION: This immune response is also initiated by the exposure to the extracellular antigen to our immune cells. At first the antigen enters into the host body and then it come in contact with the antigen presenting cells (APCs). After this exposure to the extracellular antigen the antigen is engulfed and processed by the APCs and expressed on the surface of the pathogen infected cells by forming complex with the MHC molecules.
Then the receptor (T-cell receptor) present on the surface of the naive T-cell binds with the MHC-Ag complex expressed on the surface of the infected cell or APC. This interaction differentiates the naive T-cell (both cytotoxic and T-helper cell) into effector T-cell and also induces the secretion of cytokines.
The naïve cytotoxic T-lymphocytes (CTLs) are incapable of killing target cell and are referred as cytotoxic T-lymphocyte precursor (CTL-P) which are functionally immature state of CTL. Then the interaction with the macrophages convert the naïve cytotoxic T-cell into effector T-cell. This conversion is done by the binding of the T-cell receptor (present on CTL surface) to the Ag-MHC-I complex expressed on the surface of macrophages (also an antigen presenting cell) and binding of the cytokines, secreted from the macrophages with the cytokine receptors present on the surface of the cytotoxic T-cell. After activation by the macrophages and cytokines the cytotoxic T-lymphocyte precursor become effector cytotoxic T-lymphocyte with cytotoxic activity. The effector or activated cytotoxic T-cell secretes perforin protein which form pores on the target cells and ultimately causes the lysis of the infected cell. Through this it become capable to induce apoptosis or programmed cell death.
The naïve T-helper cell is activated by the binding of the T-cell receptor to the Ag-MHC-II complex present on the surface of the antigen presenting cells and differentiated into effector T-helper cell Further the effector T-helper cell induces a specific immune response to the extracellular antigen by secreting cytokines and lymphokines.
NOTABLE: Some other markers (except the CD4 and CD8) present on the T-cell also help in this immune response as like CD28, present on the surface of the cytotoxic T-cell, CD40 present on the surface of activated T-helper cell and act as the ligand of some signalling molecule and provides an important costimulatory signal to the APC.