to tec about thymus,+ve and -ve selection - genius123

[ArchivalUser]

Precursor T cell,the stem cell is initially double negative,i,e it doesnt express CD4 or CD8.In the cortex of thymus it differentiate and express both CD4 and CD8 becoming double +ve.
these double +ve cells r present in cortex

This double +ve cell
if com into contact with MHCII=becom CD4
if com into contact with MHCI=CD8
these single +ve cells r present in medulla

before that:
those CD4+ve and CD8+ cells which r unable to react with MHC prot r killed.coz these cells wont b able to recognize any antigen presented by MHC.so we dont need them .The only T cells left r those which recognize our own MHC so this is "Positive selection"

those CD4+ and CD8+ cells which bear antigen recep for our own self prot r also killed by apoptosis.coz we dont need such cells which should react with our own prot.The removal of these self reactive cells is "Negative Selection".

with these twpo processes :
we wil hav those T cells which hav ability to react with foriegn antigen and with the help of recognition of MHC.
Both of these features r required for effective immune response.

[ArchivalUser]

Hassall's corpuscles produce cytokine,which directs the maturation of dendritic cells, and increases the ability of dendritic cells to convert naive thymocytes to a specific T cell lineage.

[ArchivalUser]

thanks alot..
but in fa there is some thing confusing me about this selection stuff ..can u please check fa page 184,under thymus topic? are they correct?

[ArchivalUser]

infront of negative selecetion in brackets they have mentione nonreactive to self
shouldnt it be self reactive? thats why they are eliminated right? then why have they mentioned nonreactive to self in the bracket? are they trying to say they are nonreactive?
very confusing..
and also infront of positive they have mentioned in the braket that its MHC restriction? can u plz clear this confusion?
thanks

[ArchivalUser]

same thing tec.it says

Positive selection=MHC restriction,means if not react with MHC,wil be killed coz no use to keep them

Negative selection=Non reactive to self,means if t cell react with our own prot,killed,we dont need them either,shouldnt react with self

[ArchivalUser]

Thanks. I got it.

[ArchivalUser]

u r welcom tec.

[ArchivalUser]

great explanation genius123.ur id matches u.
u made a confusing concept so simple

[ArchivalUser]

any T cell that enters the circulation has to react with self MHC, that is the only way TCR can react with antigen because they can respond to antigen only if antigen is associated with MHC(Self MHC). T cells in the thymus that do not recognize self MHC are killed(this is not negative selection, rather this is failure of positive selection)on the other hand if the T cells reacts in the thymus with very high affinity for self MHC, this raises the risk of auto immunity, thus this cells are also killed(this is negative selection because even though this cells are capable of reacting with MHC and thus antigens, they are killed)
this leaves the T cells with just the right amount of affinity for self MHC, (not too high, but not too low)this T cells enter circulation. this is positive selection

[ArchivalUser]

from wikepedia-

In immunology positive selection is when: developing T-cells are selected for recognition of self MHC (and self peptides). This ensures an immune response can develop. Followed by negative selection

Negative selection (immunology) T-cells that recognize self MHC molecules (and peptides) with high affinity are deleted from the repertoire of cells. Tolerance is produced. avoids auto-immunity. occurs after positive selection.