postive and negative selection

[aspiringmd1015]

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what kaplan says seems a little different from what FA says, so just trying to make this concept clear. T cells beofre entering the thymus are triple negative, and when they enter the cortex they become triple positive, depending on whether they bind mhc1 or 2 makes them cd 4 or 8 positive, and if they have the ability to bind in the cortex this is postive selection, and if they cant bind its failure of positive selection, now they test affinity in the medulla, and if affinitys too high they undergo apoptosis, and if its low affinity they survive?