For immune cells like T and B cells that means having the right ligand or surface molecule to get through the blood brain barrier, this is kind of like having the a VIP pass to get into an exclusive club.
对于免疫细胞来说,比如T淋巴细胞和B淋巴细胞,有相合配体或表面分子就可以通过血脑屏障,有点像拥有了VIP通行权才能进入私人俱乐部。