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The below mentioned article provides notes on cell-mediated (cellular) immunity.
The cell-mediated or cellular immunity is that where the T-lymphocytes destroy other cells having antigens on their surface without any mediation by antibodies. The precursors of T-lymphocytes produced by stem cells of bone marrow pass through liver and spleen before reaching the thymus where they are processed, hence called thymus-dependent (T) lymphocytes.
These lymphocytes come under the influence of the hormone “thymosin” and become immunologically competent and are called lymplioblasts. When stimulated by an appropriate antigen, the lymphoblasts divide and differentiate into cytotoxic T-lymphocyte (killer T-Iymphocytes), helper T-cells, and suppressor T-cells.
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The cytotoxic T-lymphocytes, in addition with other T-lymphocytes, release biologically active soluble factors collectively called lymphokines which act as a biochemical mediators of cellular immunity.
Unlike B-lymphocytes which are normally stimulated by free antigens in the circulatory system of the body, the cytotoxic T-lymphocytes possess specific cell surface proteins, called T-cell receptors, on their surface and respond to only major histocompatibility complex antigens (MHC-antigens) bound to the surface of other cells.
After the interaction between T-cell receptor and MHC-antigen is established and the cytotoxic T-lymphocyte cells binds the MHC-antigen containing cell, the latter undergoes lysis and is phagocytised (Fig. 41.2).
The cell-mediated immunity is important in controlling those infections where the pathogens are intracellular and reproduce within the infected cells (e.g., viruses, rickettsia, chlamydia, some protozoans like Trypanosomes, etc.).
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In such infections the antibodies (hence the antibody-mediated or humoral immunity) prove to be ineffective because the antibodies are unable to penetrate and attack intracellular pathogens multiplying within the host cells.
In addition, the cellular immunity is considered to play an important role in monitoring and regulating the proliferation of abnormal type of cells, (e.g., would be tumor cells), and thus, inhibiting the tumor development.
