The story
So named because the first identified antigen (RhD) was originally believed to be the same as one discovered in rhesus macaques.
Etymology from Wiktionary, CC BY-SA 4.0.
Rhesus system
Today it means: A blood group system, the second most important after the ABO system, and important in human blood transfusion, whereby blood types are classified according to the presence ("positive") or absence ("negative") of potentially over 50 antigens, of which the most commonly referenced is RhD. Full definition →