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.