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1550s, from French réjection (16c.) or directly from Latin reiectionem (nominative reiectio) "act of throwing back," noun of action from past participle stem of reicere (see reject). In 19c., it also could mean "excrement." Medical transplant sense is from 1954. In the psychological sense, relating to parenting, from 1931.
rejection re·jec·tion (rĭ-jěk'shən)
The act of rejecting or the state of being rejected.
The failure of a recipient's body to accept a transplanted tissue or organ as the result of immunological incompatability; immunological resistance to foreign tissue.
Note: Rejection is the most serious problem faced in surgery involving organ transplants. Drugs are used to suppress the immune system after organ transplant in order to prevent the rejection of and eventual death of the transplanted tissue.