Hi everyone,
In UW, the explanation for T-cell positive and negative selection essentially says the same thing: T-cells reactive to self are allowed to survive (positive selection) while at the same time, T-cells reactive to self are destroyed (negative selection)? Shouldn't all T-cells reactive to self be destroyed?? I don't understand this concept...if anyone has a rational explanation for this, please let me know
Thanks. (I just might be too dumb to see the obvious here!)

positive selection - T cell that have "low affinity" for Self MHC presentation are allowed to survive.

negative selection - T cell that have "high affinity" for Self MHC presentation are signalled to undergo apoptosis.


so the above two processes are completly different.

also T cells that "dont have any affinity" to presenting MHC are not allowed to survive.

i guess this should clear your doubts.

i will try to explain this topic in a very simple way....in a normal body, a MHC complex on a cell surface will be checked everytime when a T cell is arround; normally this check should be very bright: T cell will bind to that MHC for a very short time(or better said with low affinity), 'checks' if the MHC is normal(aka, opposite to what T cell has on it's surface) and then T cell will leave the cell alive and move forward and check another cells;

now,when T cells are produced this process from above should be the same but not all the T cells have this capacity; general speaking there are 2 options:
1.T cell does not bind to MHC at all-those T cell will be destroyed from the start
2.T cell will bind to a MHC complex; now here we have 2 options here:

a.T cell binds 'lightly' to MHC(meaning has low afinity).....this is the normal stuff and because this T cells are normal, they will survive; for this reason those T cells are said to aquire a positive selection(meaning they will survive)
b.T cells binds 'too tight' to MHC complex....this is abnormal and even though those cells are binding to MHC, because they are binding to tight(high affinity) are not normal and as a consequence they will be destroy; from this point of view those cells get the negative selection(meaning they don't survive)

i hope this will help you.....good luck!

Thank you very much- it's clear now!