Human blood is categorized by the presence or absence of specific proteins, called antigens, on red blood cells. The Rhesus (Rh) factor is one such inherited protein, significantly influencing blood compatibility. Understanding the Rh factor is important for medical procedures and maternal health.
Understanding Rh Negative Blood
The Rh factor is a protein, the Rh(D) antigen, found on the surface of red blood cells. Individuals with this protein are Rh-positive; those without it are Rh-negative. This distinction comes from the RHD gene: Rh-positive individuals typically have an intact RHD gene, while Rh-negative individuals have a non-functional version or a deletion. Since the Rh(D) antigen is a dominant trait, a person is Rh-negative only if they inherit two non-functional gene copies, one from each parent.
Global Distribution of Rh Negative Blood
Globally, about 6% of the population is Rh-negative, with 94% being Rh-positive. This distribution varies significantly by region and ethnic group. Europe has the highest prevalence, with 15% to 17% of Europeans having this blood type. The Basque people, in Spain and France, show the highest rates worldwide, from 21% to 36%.
Rh-negative blood is far less common elsewhere. Most of Asia, including China and Japan, reports rates under 1%. Indigenous populations in the Americas and Oceania also show very low frequencies, often below 1%. African populations vary; many sub-Saharan groups have low rates (1% to 3%), though some North African populations can reach up to 29%.
Explaining the Distribution
The uneven global distribution of Rh-negative blood is a subject of scientific inquiry. The Rh-negative allele likely originated from a random genetic mutation, a deletion within the RHD gene. Scientists estimate this mutation first appeared over 100,000 years ago in Africa, where it is present in small amounts (1% to 3%), but its prevalence significantly increased in Europe.
The rise of the Rh-negative trait in Europe is estimated to have occurred 30,000 to 35,000 years ago. This concentration is attributed to factors like genetic drift or a founder effect. The high rates among the Basque people result from their long-standing genetic isolation, which limited admixture and preserved ancient European genetic signatures.
Rh Negative Blood and Health
Having Rh-negative blood does not affect an individual’s general health, but it carries significant implications for blood transfusions and pregnancy. For transfusions, Rh-negative individuals must receive only Rh-negative blood to prevent an immune response. Rh-positive individuals can accept both Rh-positive and Rh-negative blood.
The main health consideration for Rh-negative individuals is during pregnancy, especially when an Rh-negative mother carries an Rh-positive fetus. If the mother’s blood is exposed to the fetus’s Rh-positive blood, such as during childbirth or certain medical procedures, her immune system may produce anti-D antibodies. These antibodies can cross the placenta in subsequent pregnancies with an Rh-positive fetus, attacking fetal red blood cells and potentially causing hemolytic disease of the newborn (HDFN). HDFN can lead to severe anemia, jaundice, or stillbirth. To prevent this, Rh-negative mothers usually receive an Rh immune globulin (RhoGAM) injection around 28 weeks of pregnancy and again within 72 hours after delivery if the baby is Rh-positive.