The MHC restriction of t cell is determined in positive selection. The tolerance of t cells to autoantigen is acquired during negative selection.

阳性选择决定了成熟T细胞的MHC限制性，阴性选择使机体获得自身免疫耐受。

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After controlling for self-selection bias, the impact of PBC on reliability shifted from negative to strongly positive.

FORBES: Magazine Article

The point is that a bowler like Harmison can have such an impact a positive or negative impact on his team that he will always be a tough one to discuss at the selection table.

BBC: Justin Langer column