Okay, the user wants me to translate "transplant rejection" into Simplified Chinese. Let me check if the term is already in Simplified Chinese. The original term is in English, so I need to translate it.
First, I recall that "transplant" translates to "移植" and "rejection" in this context is "排斥反应". So putting it together, it should be "移植排斥反应".
Wait, let me confirm. In medical terms, "transplant rejection" is commonly translated as "移植排斥反应" in Simplified Chinese. Yes, that's correct. I don't see any other possible translations here. The user specified to provide only the translation without explanations, so I just need to output "移植排斥反应".