What is an immune system? Write all you know about cell mediated immune system.

Explanation

The human immune system comprises lymphoid organs, tissue cells and soluble molecules such as anti-bodies. The immune system protects the body. This also plays an important role in allergic reactions, auto-immune diseases and organ transplantation.
(i) Cell-mediated Immunity (CMI). T lymphocytes, after their origin in the bone marrow, first migrate to the thymus gland. Here they divide rapidly and develop extreme diversity for reacting against different specific antigens. These different types of processed T lymphocytes now leave the thymus and like B lymphocytes migrate to the lymphoid tissue throughout the body.
The cell mediated immunity is the responsibility of cytotoxic T cells (a sub group of T cells) ruther than B cells and antibodies. An activated cytotoxic T cell is specific to a target cell that has been infected, and kill the target cell. This prevents the completion of life cycle of the pathogen because it depends on an intact host cell. Cytotoxic T cells also kill the cancer cells.
The first description of a CMI response was the observation by Jenner (1798). The immune response to tuberculin (protein extract of tubercle bacillus) was observed by Robert Koch in 1890. Thereafter the tuberculin test became a general model or example for delayed hypersensitiviiy (DH). The tern delayed hypersensitivity refers to the appearance of skin lesion 48-72 hours after adminstration of the antigen. The cellular basis of delayed hypersensitivity was shown by Landsteiner and Chase (1942).
Biologically active substances released by activated T lymphocytes are called lymphokines. They are regulatory proteins whose main function appears to be regulation of immune response and growth and functions of cells of the reticuloendothelial system, t.g., macrophages, phagocytic macrophages which are present in linings of sinuses and in reticulum of various organs and tissue (spleen, liver, lymph nodes, bone marrow connective tissue, etc.)
CMI protects against fungi, most of the viruses and intracellular bacterial pathogens like M. leprae, M. tuberculosis and Salmonella and parasites like Leishmania and Trypanosomas. It also participates in allograft rejection, graft versus host reaction, delayed hypersensitivity and certain autoimmune diseases. It provides immunity against cancer.