Immune Self Tolerance - An Overview

 Immune system has the ability to react with enormous and diverse foreign antigens, While not reacting or mounting immune responses to the self antigens.
  This means our immune system tolerates the self-antigens.

The state of unresponsiveness of the immune system to a particular antigen is known as immune or immunological tolerance. This means the immune system does not mount an immune response when it encounters the antigen.

  Antigen is a substance that is recognized by our immune system as foreign particle. And as a result an immune response is stimulative against it.

Antigens can be foreign or self.
• Foreign antigens or hetero antigens are not part of our body. For example bacteria, viruses, pollen, parasites etc.
• Self-antigens are components of our own body. For example our own cells and cell surface molecules.

The state of unresponsiveness of the immune system to self antigens is known as self tolerance.
If self tolerance fails or The immune system starts responding to self antigens, it leads to autoimmunity. That means immune system starts attacking self. Further this may result in autoimmune diseases, In which individuals own molecules cells, tissues and organs are damaged as a result of immune system attack.

   The main function of our immune system is to provide protection against the invasion of foreign antigens. For this function we have three lines of defense in our body namely :
- First line of defense
- Second line of defense and
- Third line of defense

The Third line of defense is the most significant. It is made up of B and T lymphocytes, which are responsible for the adaptive immunity.
• The B cells and T cells Express antigen-specific receptors namely B-cell receptor and T-cell receptor.
• B and T cell receptors are encoded by random rearrangements of pre-existing gene segments and genes.
• This process occurs in the early development of lymphocytes.
• These antigen specific receptors of B and T cells are generated prior to the encounter of lymphocytes with the specific antigens.
• Our immune system generates lymphocytes clones that can virtually recognize any antigen in the universe. 
  This also means that T and B cells having receptors against self antigens are also being generated in our body. These lymphocytes are known as autoreactive or self reactive lymphocytes.

How immune responses to self antigens are prevented in human body

  State of unresponsiveness to self antigens or self tolerance is achieved by various mechanisms and processes operating on the cells of the immune system.
  Establishment of self tolerance can be divided into a two-step process
- First step is known as central tolerance and
- Second step is known as peripheral tolerance

Central tolerance
   The term central refers to the primary or central lymphoid organs, where development of lymphocytes takes place.
• These organs are the bone marrow and thymus. These are the sites where T and B lymphocytes to express their antigen receptors.
• These organs provide the growth factors and other molecular signals for lymphocyte maturation.
• Central tolerance is the tolerance induced during early development of lymphocytes.
   The mechanisms and processes of central tolerance eliminate most autoreactive B and T cells during their early development. But central tolerance is not foolproof. This is where role of peripheral tolerance comes into play.

Peripheral tolerance
  The term peripheral refers to the secondary or peripheral lymphoid organs.
• These include lymph nodes spleen and the components of the mucosal immune system.
• The main function of secondary or peripheral lymphoid organs is to capture and concentrate pathogens.
• These organs are the sites where circulating lymphocytes come in contact with the antigens and lymphocyte responses to foreign antigens are initiated.
   So the autoreactive lymphocytes that escape the central tolerance will be found here.
• Peripheral tolerance is the tolerance induced in mature lymphocytes. The mechanisms and process of peripheral tolerance prevent the autoreactive mature lymphocytes from attacking self antigens in the cunning.  

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