Role of Antigen Presenting Cells
When foreign organisms invade the body, the macrophages or other antigen presenting cells kill them mostly by means of phagocytosis. Later, the antigen from the organisms is digested into polypeptides. The polypeptide products are presented to T lymphocytes (and also to В lymphocytes) along with human leukocyte antigens (HLAs). HLAs are the molecules arranged in series in the genes located in short arm of chromosome 6. The cluster of genes with HLA is called major histocompatibility complex (MHC). HLAs are easily recognized by the cells of immune systems and hence, the name antigen is given to them.
Now, the antigenic products activate the helper T cells and В lymphocytes. The macrophages also secrete some substance called interleukin 1. This causes activation and proliferation of lymphocytes.
The activated helper T cells are proliferated and released into circulation from lymphoid tissues.
ROLE OF HELPER T CELLS
Helper T cells help in promoting various activities of immune system. Activated by the antigenic products of foreign body the helper T cells stimulate the other T cells and the В cells.
There are two types of helper T cells called T helper-1 (TH1) cells and T helper-2 (TH2) cells.
Role of TH1 Cells
TH1 cells are concerned with cellular immunity. These helper cells secrete interleukin 2 and gamma interferon Interleukin 2 activates the other T cells. Gamma interferon promotes phagocytic action of cytotoxic cells, macrophages and natural killer (NK) cells.
Role of TH2 Cells
TH2 cells are concerned with humoral immunity and secrete interleukin 4 and interleukin 5. These two interleukins are concerned with:
Activation of В lymphocytes
Proliferation of plasma cells
Production of antibodies by plasma cell. Interleukin4 induces the secretion of IgE and interleukin 5 induces the secretion of IgA.
ROLE OF CYTOTOXIC T CELLS
The activated cytotoxic T cells circulate though blood lymph and lymphatic tissues and destroy the invading organism by attacking them directly.
Mechanism of Action of Cytotoxic T Cells
The outer membrane of cytotoxic T cells contains some receptor proteins. These receptor proteins bind the antigens or organisms tightly with cytotoxic T cells. Then, the T cells are enlarged and release cytotoxic substances like the lysosomal enzymes. These substances destroy the invaded organisms. Like this, each killer cell can destroy a large number of microorganisms one after another.
Other Actions of Cytotoxic T Cells
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The cytotoxic T cells also destroy cancer cells, transplanted cells like those of transplanted heart or kidney or any other cells, which are foreign bodies.
Cytotoxic T cells may destroy even the tissues affected by the foreign bodies particularly the viruses. Man) viruses are entrapped in the membrane of affected cells. The antigen of the viruses attracts the T cells. And the cytotoxic T cells kill the affected cells also along with viruses. So, the name killer cell is obtained.
ROLE OF SUPPRESSOR T CELLS
The suppressor T cells are also called regulatory T cells. These T cells suppress the activities of the killer T cells. Thus, the suppressor T cells play an important role in preventing the killer T cells from destroying the body's own tissues along with invaded organisms. The suppressor cells suppress the activities of helper T cells also.
ROLE OF MEMORY T CELLS
Some of the T cells activated by an antigen remain in lymphoid tissue instead of entering circulation. These T cells are called memory T cells.
In later periods, the memory cells migrate to various lymphoid tissues throughout the body. When the body is exposed to the same organism for the second time, the memory cells identify the organism and immediately activate the other T cells. So, the invading organism is destroyed very quickly. The response of the T cells is also more powerful this time.
SPECIFICITY OF T CELLS
Each T cell is designed to be activated only by one type of antigen. It is capable of developing immunity against that antigen only. This property is called the specificity of T cells.
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