rhesus factor Definition

rhesus factor
Rh factor
noun

1. medicine.
An antigen that is present on the surface of red blood cells of about 84% of the human population, who are said to be rhesus positive, and absent in the remaining 16%, who are said to be rhesus negative. It is necessary to discover whether someone is Rhesus positive or negative before a blood transfusion.

Etymology: 1940s: named after the rhesus monkey, in which it was first discovered.