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Signal 2 only occurs after the recognition of <font color=purple>DANGER</font>.
 
Signal 2 only occurs after the recognition of <font color=purple>DANGER</font>.
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===Scenarios===
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===Activation Scenarios===
 
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#'''No signal 1''':
* '''No signal 1'''
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*T cell is not activated as there is no antigen.
** [[T cells|T cells]] is not activated as there is no antigen.
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#'''Both signal 1 and signal 2'''
* '''Both signal 1 and signal 2'''
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*T cell is activated into clonal expansion and produces cytokines or becomes cytotoxic.  
** [[T cells|T cells]] is activated into clonal expansion.
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#'''Signal 1 but no signal 2'''
*** Produces cytokines or becomes cytotoxic.  
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* T-cell is triggered into apoptosis and dies.  
** This is the response to a dangerous antigen.
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* This is the basis of "clonal deletion" and is a major mechanism of the development of tolerance.  
* '''Signal 1 but no signal 2'''
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It ensures that T-cells do not react with self (non-dangerous) antigens.
** T-cell is triggered into apoptosis and dies.  
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** This is the basis of "clonal deletion".
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*** A major mechanism of tolerance.  
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*** Ensures that T-cells do not react with self (non-dangerous) antigens.
      
===TCR Complex===
 
===TCR Complex===
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