Immunity, Cell-Mediated
The immune system is a network of cells and organs that work together to protect the body from infectious organisms. Many different types of organisms such as bacteria, viruses, fungi, and parasites are capable of entering the human body and causing diseases. It is the immune system's job to recognize these agents as foreign and destroy them.
The immune system can respond to the presence of a foreign agent in one of two ways. It can either produce soluble proteins called antibodies, which can bind to the foreign agent and mark them for destruction by other cells. This type of response is called a humoral response or an antibody response. Alternately, the immune system can mount a cell-mediated immune response. This involves the production of special cells that can react with the foreign agent. The reacting cell can either destroy the foreign agents, or it can secrete chemical signals that will activate other cells to destroy the foreign agent.
During the 1960s, it was discovered that different types of cells mediate the two major classes of immune responses. The T lymphocytes, which are the main effectors of the cell-mediated response, mature in the thymus, thus the name T cell. The B cells, which develop in the adult bone marrow, are responsible for producing antibodies. There are several different types of T cells performing different functions. These diverse responses of the different T cells are collectively called the "cell-mediated immune responses".
There are several steps involved in the cell-mediated response. The pathogen (bacteria, virus, fungi, or a parasite), or foreign agent, enters the body through the blood stream, different tissues, or the respiratory tract. Once inside the body, the foreign agents are carried to the spleen, lymph nodes, or the mucus-associated lymphoid tissue (MALT) where they will come in contact with specialized cells known as antigen-presenting cells (APC). When the foreign agent encounters the antigen-presenting cells, an immune response is triggered. These antigen presenting cells digest the engulfed material, and display it on their surface complexed with certain other proteins known as the Major Histocompatibility Class (MHC) class of proteins.
Next, the T cells must recognize the antigen. Specialized receptors found on some T cells are capable of recognizing the MHC-antigen complexes as foreign and binding to them. Each T cell has a different receptor in the cell membrane that is capable of binding a specific antigen. Once the T cell receptor binds to the antigen, it is stimulated to divide and produce large amounts of identical cells that are specific for that particular foreign antigen. The T lymphocytes also secrete various chemicals (cytokines) that can stimulate this proliferation. The cytokines are also capable of amplifying the immune defense functions that can eventually destroy and remove the antigen.
In cell-mediated immunity, a subclass of the T cells mature into cytotoxic T cells that can kill cells having the foreign antigen on their surface, such as virus-infected cells, bacterial-infected cells, and tumor cells. Another subclass of T cells called helper T cells activate the B cells to produce antibodies that can react with the original antigen. A third group of T cells called the suppressor T cells are responsible for regulating the immune response by turning it on only in response to an antigen and turning it off once the antigen has been removed.
Some of the B and T lymphocytes become "memory cells," that are capable of remembering the original antigen. If that same antigen enters the body again while the memory cells are present, the response against it will be rapid and heightened. This is the reason the body develops permanent immunity to an infectious disease after being exposed to it. This is also the principle behind immunization.
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