noun
also rh äˈrāch-, ȧˈ(r)āch-
Usage: usually capitalized R
Etymology: rh esus monkey (in which it was first detected)
: a substance or one of a group of substances that is present in the red blood cells of a large majority of persons and in those of other higher animals, is prob. an agglutinogen, is inherited according to Mendelian principles, and is capable of inducing intense antigenic reaction under suitable circumstances (as in repeated transfusion of Rh-positive blood to an Rh-negative person) — called also Rh antigen, rhesus factor ; compare hr factor