rhesus factor
noun/ˈriːsəs fæktə(r)/
/ˈriːsəs fæktər/
[singular] (medical)- a substance present in the red blood cells of around 85% of humans. These people are rhesus positive and cannot share blood (through blood transfusions) with people who are rhesus negative (= do not have the substance in their blood).Word Origin1940s: rhesus from rhesus monkey, in which the antigen was first observed.
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rhesus factor