Changes

Activation of [[Lymphocytes#T cells|T cells]] requires two distinct signals:  

Activation of [[Lymphocytes#T cells|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.  

*'''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.

**** APC expression of CD80 only occurs after:

Signal 2 only occurs after the recognition of <font color=purple>DANGER</font>.

***** The engagement of pattern recognition or Fc receptors.

***** Activation with cytokines.

****** Interferons, IL-1&beta; or TNF&alpha;.

**** Therefore '''signal 2 only occurs after the recognition of <font color=red>danger</font>'''.

===Scenarios===

===Scenarios===