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→T-Cell Activation
==T-Cell Activation==
==T-Cell Activation==
T cells function only after recent activation by an antigen.
T cells function only after recent activation by an antigen.
* 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<sup>+</sup> 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<sup>+</sup> T-cells]] recognise antigen only in association with [[Major Histocompatability Complexes#MHC I|MHC class I]].
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.
===Response to Activation===
===Response to Activation===
The response of the T cells to obtaining Signals 1 and 2 is to express the receptor for the cytokine interleukin-2 (IL-2) and CD4+ T-cells secrete IL-2.
The response of the T cells to obtaining Signals 1 and 2 is to express the receptor for the cytokine interleukin-2 (IL-2) and CD4<sup>+</sup> T-cells secrete IL-2.
The final trigger for clonal expansion is the engagement of IL-2R with IL-2 from any activated CD4+ T-cell. IL-2 produced by a CD4+ cell may also stimulate clonal expansion of the CD4+ cell.
The final trigger for clonal expansion is the engagement of IL-2R with IL-2 from any activated CD4+ T-cell. IL-2 produced by a CD4<sup>+</sup> cell may also stimulate clonal expansion of the CD4<sup>+</sup> cell.
==T-Helper Cell Function==
==T-Helper Cell Function==