Rhesus is a term used in biology to denote a genus of Old World monkeys and, in medicine, of a blood-group system (as in RhD). In zoology, it often refers to the rhesus macaque, a common research model. In medicine, “Rhesus” is encountered in phrases like Rh incompatibility, spelled with a capital R and sometimes pronounced as /ˈriːəs/ in some contexts.
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