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* CD4 binds MHC class II - [[T_cells#Helper_CD4.2B|CD4+ T-cells]] recognise antigen only in association with [[Major Histocompatability Complexes#MHC II|MHC class II]].  
 
* CD4 binds MHC class II - [[T_cells#Helper_CD4.2B|CD4+ T-cells]] recognise antigen only in association with [[Major Histocompatability Complexes#MHC II|MHC class II]].  
 
* CD8 binds MHC class I - [[T_cells#Cytotoxic_CD8.2B|CD8+ T-cells]] recognise antigen only in association with [[Major Histocompatability Complexes#MHC I|MHC class I]].  
 
* CD8 binds MHC class I - [[T_cells#Cytotoxic_CD8.2B|CD8+ T-cells]] recognise antigen only in association with [[Major Histocompatability Complexes#MHC I|MHC class I]].  
* Activation of [[Lymphocytes#T cells|T cells]] requires two distinct signals.
+
Activation of [[Lymphocytes#T cells|T cells]] requires two distinct signals:
** '''Signal 1'''
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* '''Signal 1''' is the interaction of the TcR with the antigenic peptide/[[Major Histocompatability Complexes|MHC]] complex on the antigen presenting cell.  
*** The interaction of the TcR with the antigenic peptide/[[Major Histocompatability Complexes|MHC]] complex on the antigen presenting cell.  
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*'''Signal 2''' is the interaction of CD28 on the T cells with its ligand, CD80, on the antigen-presenting cell.  
** '''Signal 2'''
  −
*** The interaction of CD28 on the [[Lymphocytes#T cells|T cells]] with its ligand, CD80, on the antigen-presenting cell.  
   
**** APC expression of CD80 only occurs after:
 
**** APC expression of CD80 only occurs after:
 
***** The engagement of pattern recognition or Fc receptors.  
 
***** The engagement of pattern recognition or Fc receptors.  
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