/ ˈriːsəs fæktə(r); NAmE / noun
[ sing. ] ( medical ) 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 .
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WORD ORIGIN
1940s: rhesus from rhesus monkey , in which the antigen was first observed.