The story
The antigen was discovered in connection with a research of rhesus monkeys.
Etymology from Wiktionary, CC BY-SA 4.0.
Rhesus factor
Today it means: An antigen of the Rhesus system; understood unless otherwise specified to refer to Rhesus factor D (RhD), which appears on the surface of the red blood cells of most (85%) humans. An incompatibility between the Rh-blood types of a mother and her baby may cause serious disease for the baby. Full definition →