Changes

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>.

===Scenarios===

===Activation Scenarios===

 

#'''No signal 1''':

* '''No signal 1'''

*T cell is not activated as there is no antigen.

** [[T cells|T cells]] is not activated as there is no antigen.

#'''Both signal 1 and signal 2'''

* '''Both signal 1 and signal 2'''

*T cell is activated into clonal expansion and produces cytokines or becomes cytotoxic.  

** [[T cells|T cells]] is activated into clonal expansion.

#'''Signal 1 but no signal 2'''

*** Produces cytokines or becomes cytotoxic.  

* T-cell is triggered into apoptosis and dies.  

* This is the basis of "clonal deletion" and is a major mechanism of the development of tolerance.  

* '''Signal 1 but no signal 2'''

It ensures that T-cells do not react with self (non-dangerous) antigens.

** T-cell is triggered into apoptosis and dies.  

** This is the basis of "clonal deletion".

*** A major mechanism of tolerance.  

*** Ensures that T-cells do not react with self (non-dangerous) antigens.

===TCR Complex===

===TCR Complex===