Definition of rhesus factor noun from the Oxford Advanced Learner's Dictionary

 

rhesus factor

 noun
noun
BrE BrE//ˈriːsəs fæktə(r)//
 
; NAmE NAmE//ˈriːsəs fæktər//
 
[singular](medical)
 
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a substance present in the red blood cells of around 85% of humans. Its presence (rhesus positive) or absence (rhesus negative) can be dangerous for babies when they are born and for people having blood transfusions. Word Origin 1940s: rhesus from rhesus monkey, in which the antigen was first observed.
See the Oxford Advanced American Dictionary entry: rhesus factor