Definition of Rhesus factor in US English:

Rhesus factor

(also Rh factor)


  • An antigen occurring on the red blood cells of many humans (around 85 percent) and some other primates. It is particularly important as a cause of hemolytic disease of the newborn and of incompatibility in blood transfusions.

    • ‘Treatment for women with rhesus factor disease (where the Rh blood groups of the mother and baby do not match) is essential.’
    • ‘Anti-D injections are used to counteract the rhesus factor if it proves necessary after a blood test.’
    • ‘The rhesus factor protein is the next most important one after the A and B proteins.’
    • ‘There are also subsidiary blood proteins such as the rhesus factor that can be positive or negative (absent).’
    • ‘The discovery of the rhesus factor in human blood by American scientists also had important implications for blood grouping and, ultimately, for the development of human genetics.’


1940s: rhesus from rhesus monkey, in which the antigen was first observed.


Rhesus factor

/ˈrisəs ˌfæktər//ˈrēsəs ˌfaktər/