What is the Mechanism of Cell Mediated Immunity?


The mechanism of cell mediated immunity includes:

(1) Activation of antigen-specific cytotoxic T-lymphocytes that is able to induce apoptosis in body cells displaying epitopes of foreign antigen on their surface such as cells with intracellular bacteria, virus-infected cells and cancer cells displaying tumor antigens.

(2) Activating macrophages and natural killer cells, enabling them to destroy intracellular pathogens.


(3) Stimulating cells to secrete a variety of cytokines that influence the function of other cells involved in immune responses.

Irrespective of the type of cells involved in CMI reaction, the complete mechanism of cell mediated immunity proceeds in three phases.

(a) Binding of the cytotoxic cells to the target cells (b) Release of cytokines (c) Lysis of the target cell.

(a) Binding of the cytotoxic cells to the target cells:


Close binding of the cytotoxic cells with the target cells is essential to execute the action of lysis. The cytotoxic cells bind to the target either through antibody or MHC class I molecules or by the receptors.

(b) Release of cytokines:

Following binding with the target cells, the cytotoxic cells release their secretions. The secretions (cytokines) contain perforin and other enzymes that can cause apoptosis like reactions in target cells.

(c) Lysis of the target cell:


The perforin enzyme present in released cytokines binds to the cell in the presence of Ca+, and develops Tran’s membrane channels (pores). The remaining cytotoxic products pass through the pores and cause fragmentation of nucleus, followed by disintegration of cell in to small membrane bound vesicles.

TNFa of released cytokines play an important role in killing tumor cells. It acts via receptors and the susceptible cell dies after 3-4 hours. Subsequent to killing the target cell, the cytotoxic cell remains active and continues its lytic activity to kill some more target cells.

They protect themselves from the action of released lytic enzymes by means of proteoglycan present in them. It prevents pore forming activity of released lytic enzymes.

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