Rh factor

1942, from the first letters of rhesus; so called because the blood group, and its effects, were discovered in the blood of rhesus monkeys (1941).

Others are reading

Definitions of Rh factor from WordNet

Rh factor (n.)
a blood group antigen possessed by Rh-positive people; if an Rh-negative person receives a blood transfusion from an Rh-positive person it can result in hemolysis and anemia;
Synonyms: rhesus factor / Rh