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Activation of T cells requires two distinct signals:  
 
Activation of T cells requires two distinct signals:  
 
* '''Signal 1''' is the interaction of the TcR with the antigenic peptide/[[Major Histocompatability Complexes|MHC]] complex on the antigen presenting cell.  
 
* '''Signal 1''' is the interaction of the TcR with the antigenic peptide/[[Major Histocompatability Complexes|MHC]] complex on the antigen presenting cell.  
*'''Signal 2''' is the interaction of CD28 on the T cells with its ligand, CD80, on the antigen-presenting cell (APC). APC expression of CD80 only occurs after the engagement of pattern recognition on Fc receptors or activation by the cytokines Interferon, IL-1&beta or TNF&alpha.
+
*'''Signal 2''' is the interaction of CD28 on the T cells with its ligand, CD80, on the antigen-presenting cell (APC). APC expression of CD80 only occurs after the engagement of pattern recognition on Fc receptors or activation by the cytokines Interferon, IL-1β or TNFα.
 
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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