Quick answer
Rh factor refers to the RhD protein on red blood cells. If your cells have it you are Rh-positive (+); if not, Rh-negative (−). Rh-positive is dominant: a child is Rh-negative only if they inherit a “d” allele from both parents. About 85 percent of people in the United States are Rh-positive.
What the Rh factor is
The Rh blood group system is the second most important after ABO. It contains around 50 antigens, but by far the most significant is D. “Rh-positive” means the D antigen is present on your red cells; “Rh-negative” means it is absent. The name comes from early experiments with Rhesus monkeys, though the human antigen turned out to be different.
Unlike ABO, Rh-negative people do not automatically make antibodies against D. They only develop anti-D antibodies after exposure to Rh-positive red cells, most commonly through transfusion or pregnancy. Once sensitised, however, the antibodies persist and can cause serious reactions on later exposure.
How Rh factor is inherited
Rh status follows a simple dominant/recessive pattern. Each person has two copies of the RHD gene region, written D (functional) or d (deleted or inactive). One copy comes from each parent.
| Genotype | Rh status | Notes |
|---|---|---|
| DD | Positive | Always passes D to children |
| Dd | Positive | Carrier: may pass D or d |
| dd | Negative | Always passes d |
Two Rh-positive parents who are both Dd carriers have a 25 percent chance of an Rh-negative child. An Rh-negative parent (dd) and an Rh-positive DD parent will have only Rh-positive children. Two Rh-negative parents will have only Rh-negative children.
| Parents | Possible children | Chance of Rh-negative child |
|---|---|---|
| + × + | + or − | 0% if either is DD; 25% if both are Dd |
| + × − | + or − | 0% if positive parent is DD; 50% if Dd |
| − × − | − only | 100% |
How common is Rh-negative?
Roughly 15 percent of people in the United States and Western Europe are Rh-negative, but the proportion varies widely around the world: it is highest among Basque populations (around 30 percent) and very low in much of East Asia (under 1 percent). Because Rh-negative blood can be given to Rh-negative and Rh-positive recipients alike, Rh-negative donors are in high demand.
Why Rh factor matters
Transfusion: Rh-negative patients should receive Rh-negative red cells to avoid becoming sensitised. In emergencies where the recipient's type is unknown, O-negative red cells are used because they carry neither A, B nor D antigens.
Pregnancy: if an Rh-negative mother carries an Rh-positive baby, some of the baby's red cells can enter her bloodstream and trigger anti-D antibodies. This rarely harms a first pregnancy but can affect later Rh-positive babies. Routine screening and Rh immunoglobulin (RhoGAM) injections have made this highly preventable. Read the full Rh factor and pregnancy guide.