rhesusfactor

The rhesus factor, or Rh factor, is a protein antigen found on the surface of red blood cells. The presence or absence of the D antigen defines whether a person is Rh-positive or Rh-negative. The Rh blood group system also includes other antigens such as C, c, E, and e, which are determined mainly by the RHD and RHCE genes on chromosome 1.

Historically, the term rhesus factor derives from studies with the rhesus macaque, where a similar antigen was first described in 1940 by Karl Landsteiner and Alexander S. Wiener. The human D antigen is related to this system but is a distinct component of human blood typing.

Clinical significance includes the risk of Rh incompatibility during pregnancy. If an Rh-negative mother carries an Rh-positive fetus, maternal immune responses can produce anti-D antibodies that may cross the placenta and cause hemolytic disease of the fetus and newborn (HDFN) in subsequent pregnancies. Prevention relies on Rh immune globulin (RhIg) prophylaxis given to Rh-negative pregnant individuals during pregnancy and after delivery, as well as after events that could cause fetomaternal hemorrhage.

In transfusion medicine, Rh(D) compatibility is essential for safe blood transfusions. ABO compatibility is determined first, followed by Rh(D) typing and crossmatching. Some individuals have weak D or partial D variants that can complicate serologic typing; such cases may be treated as Rh-positive to minimize alloimmunization risk.

Global prevalence varies, but approximately 85% of people are RhD-positive, with geographic and ethnic variation in the distribution of Rh types.