The classification of human blood is primarily based on two systems: the ABO blood group and the Rhesus (Rh) blood group system. The Rh system is the second most medically significant classification, determining whether an individual’s blood type is designated as positive or negative. To be RhD positive means that the red blood cells possess a specific protein, known as the D antigen, on their surface. This status is a fundamental biological characteristic that has implications for medical procedures such as blood transfusions and pregnancy management.
Defining the Rhesus D Factor and Its Prevalence
The Rhesus D factor is a protein antigen located on the outer membrane of the red blood cells. This specific protein is the product of the RHD gene, which is inherited from one’s biological parents. A person is considered RhD positive if they inherit at least one functional copy of the RHD gene, resulting in the presence of the D antigen. Individuals who are RhD negative lack this protein entirely.
The RhD positive status is the most common blood type characteristic across the global population, with approximately 85% of people worldwide possessing the D antigen. The positive or negative suffix attached to an ABO blood type, such as A+ or O-, always refers to the presence or absence of this specific D antigen.
RhD Positive Status in Blood Transfusions
The RhD status plays a direct role in ensuring the safety of blood transfusions. Since RhD positive individuals naturally carry the D antigen, their immune system recognizes it as a normal component of the body. Consequently, RhD positive patients can safely receive red blood cell transfusions from either RhD positive or RhD negative donors.
The primary concern, however, lies in the donation of RhD positive blood to an RhD negative recipient. If RhD positive red cells are given to an RhD negative person, the recipient’s immune system will identify the D antigen as foreign. This exposure can trigger the production of anti-D antibodies, which can lead to a severe transfusion reaction upon subsequent exposure to RhD positive blood. For this reason, RhD positive blood components are ideally reserved for RhD positive recipients to conserve the limited supply of RhD negative blood.
The Importance of Rh Status During Pregnancy
The RhD status gains particular medical significance when considering pregnancy, especially when the mother is RhD negative. An RhD positive status in the mother is generally uncomplicated, as her body already recognizes the D antigen. The potential for sensitization arises when an RhD negative mother carries an RhD positive fetus, who inherited the D antigen from the father.
During birth, or certain procedures and events during pregnancy, a small amount of the baby’s RhD positive red blood cells may enter the mother’s bloodstream. The mother’s immune system then mounts a response, creating anti-D antibodies directed against the D antigen. This initial sensitization usually does not affect the first RhD positive pregnancy because the exposure typically occurs late or at delivery.
However, once sensitized, the mother’s anti-D antibodies can cross the placenta in future RhD positive pregnancies. These antibodies attack and destroy the fetal red blood cells, a condition known as Hemolytic Disease of the Fetus and Newborn (HDFN). HDFN can cause severe anemia, jaundice, and even death for the fetus or newborn.
Modern medicine effectively manages this risk through preventative treatment using Rh immune globulin, often referred to as RhoGAM. This medication is a purified solution of anti-D antibodies given to the RhD negative mother at scheduled times, typically around 28 weeks of gestation and again after the delivery of an RhD positive baby. The injected anti-D antibodies bind to and neutralize any fetal RhD positive red cells that have entered the mother’s circulation before her own immune system can begin producing permanent anti-D antibodies. This prophylactic measure has dramatically reduced the incidence of HDFN caused by RhD incompatibility.