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T cells

T cells are like the police chiefs of white blood cells.

T cells are white blood cells called lymphocytes. 

There are many different types of T cells that all perform different functions, but among the most important are Helper T cells and Killer T cells. Both of these types of cells bind cells in the body and can identify whether the cell is healthy and belongs to the body or whether it's not healthy or doesn't belong. In the latter two cases, the T cell will start an immune reaction to destroy the unhealthy cell or germ. T cells can identify which particular illness they are dealing with (they are specific) and can share this information with other immune cells, so they are like the "police chiefs" of white blood cells.

Read on about the details of T cell specificity >>
T cells become Helpers or Killers during maturation >>
Read on about how T cells bind cells and what makes T cells specific >>

Helper T cells

In diseases like AIDS (Acquired Immune Deficiency Syndrome) the HIV virus attacks the Helper T cells, affecting Killer T cells and the production of antibodies by B cells.  This is one reason why the AIDS virus is so devastating.  It shuts down the highly specialized acquired defenses and leaves the body open to infection.

Helper T cells are the mediators of the immune system, they carry information and decide when to give the green light to other immune cells to carry out an immune response. 

Helper T cells can only bind to swallowing cells like the macrophage and the B cell. The macrophage has a special binding site that helps the Helper T cell grab on (learn more about MHC II ).

Acquired Immune Response

T cells never work alone, they always work in cooperation with other immune cells, which allows our immune system a level of deliberation or control in how it reacts.

Activation of Helper T cells

The first step in an immune reaction that involves T cells is the activation of helper T cells.

  1. 1 - When a macrophage engulfs and digests a particle, it displays some of the broken down proteins (antigens) on its cell surface.  
  2. 2 - Helper T cells can grab on to the macrophage by a special binding site - the MHC class III complex.
  3. 3 - Then, the T cell can attempt to “read” the antigen on the macrophage and tell what kind of particle the macrophage has eaten!
  4. 4 - If the Helper T cell thinks it is a self antigen it will drift away.
  5. 5 - If the Helper T cell recognizes the antigen as belonging to something harmful and becomes activated. 
  6. 6 - The activated Helper T cell also activates the macrophage to continue to break down that specific antigen.  The macrophage goes into overdrive, replicating and carrying out an aggressive attack on the unwanted microbe. 
  7. 7 - Once activated, the Helper T cell starts to divide and produce special proteins to signal other immune cells. The activated Helper T cell will in turn activate other T cells to attack the specific antigen (see below >>). It also can go on to activate B cells and create an antibody-mediated response. Learn more about antibody production in the following section on B cells >>
  8. 8 - Some of the newly replicated T cells will not engage in the immediate response but will remain in the body as memory T cells, maintaining their specific receptor in the body, and remembering that harmful particle.

Killer T cells

As we’ve said, once a Helper T cell is activated, it goes on to activate other cells.  One of these cells is the Killer T cell.  These cells are also called Cytotoxic T cells.  Unlike the macrophages that swallow their prey, Cytotoxic T cells kill the infected cell by injecting it with special enzymes that destroy its nucleus and/or its structure.  In scientific terms, this is called apoptosis. Cytotoxic T cells are very good at attacking cells that have been infected with viruses or bacteria, and also cancerous cells.  Normal body cells have special binding sites for Killer T cells (the MHC I). They cannot attack bacteria that live outside of the cell—that function is left for the macrophages and other phagocytes.

Killer T cell action

  1. Cells in the body are subject to infection. When this happens, the infected cell will display bits of the foreign particle on its cell membrane - nonself antigens.
  2. A Cytotoxic T cell can recognize and bind to the antigen. 
  3. Whenever T cells react with infected cells, they release special groups of proteins and peptides called cytokines.  Cytokines are like messengers; they signal other T cells and macrophages to rush to the infected area. 
  4. If a macrophage-activate Helper T cell comes by, it sends chemical signals that activate the Killer T cell.
  5. The activated killer T cell kills the infected cell.
  6. Afterwards, macrophages help eat up the residue of the dead cell.
  7. Both activated Helper and Killer T cells go on to replicate themselves and continue to attack that specific intruder. 
  8. Some Killer T cells will also remain in the body after the reaction as memory cells,  to retain information about the virus or bacteria it encountered, so next time the same particle enters the body they will recongnize it and set off an immune response. Learn more about the role of memory in the immune system >>

Replication, activation and signaling are all part of a specific immune response!

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