Tissue rejection was a medical term that referred to the incompatibility of differing body tissue when transplanted. Mehendri Solon overcame this problem, an ability he later used when restoring Morbius. (TV: The Brain of Morbius) The same issue was faced on Thoros Beta by the surgeon Crozier, in his experiments of brain transference on order of Kiv. He had perfected a serum to avoid the reaction; he claimed to be able to "put any brain in any body anywhere". (TV: Mindwarp)