Rh factor is a protein found on the surface of red blood cells. If your red blood cells carry this protein, you’re Rh-positive. If they don’t, you’re Rh-negative. About 85% of people are Rh-positive. Your Rh status is one half of your blood type label: when you see “A positive” or “O negative,” the positive or negative part refers to the Rh factor.
What the Rh Protein Actually Does
The “Rh” in Rh factor stands for Rhesus, named after the Rhesus monkey whose red blood cells were used in the experiments that first identified the protein in the 1930s and 1940s. The protein itself is embedded in the membrane of your red blood cells, threading through it 12 times like a stitch. It sits alongside a partner protein called RhAG, and together they form a complex on the cell surface.
Scientists still aren’t entirely sure what the Rh protein’s main job is. The best evidence suggests it helps transport ammonium or carbon dioxide across the red blood cell membrane and keeps red blood cells structurally sound. People whose red blood cells lack the Rh protein entirely tend to have abnormally shaped cells, which points to a role in maintaining cell integrity. But in everyday terms, your Rh status has no effect on how you feel or how your body functions day to day. It only becomes medically significant in two situations: blood transfusions and pregnancy.
How You Inherit Your Rh Status
Your Rh factor is determined by a single gene you inherit from each parent, one copy from your mother and one from your father. The Rh-positive version of the gene is dominant, meaning it overrides the negative version. So you only end up Rh-negative if you inherit the negative version from both parents.
Here’s how that plays out in practice:
- Both parents carry two positive copies (+ +): Every child will be Rh-positive.
- One parent is + + and the other is – -: Every child will be Rh-positive, but each will carry one hidden negative copy (+ -).
- Both parents are carriers (+ -): There’s roughly a 75% chance a child will be Rh-positive and a 25% chance of Rh-negative.
- Both parents are Rh-negative (- -): Every child will be Rh-negative.
This is why two Rh-positive parents can have an Rh-negative child. If both happen to carry one hidden negative copy, a child who inherits the negative copy from each parent will be Rh-negative. It’s the same basic inheritance pattern as blue eyes from two brown-eyed parents.
Why Rh Factor Matters in Pregnancy
The most well-known concern with Rh factor involves pregnancy. If you’re an Rh-negative mother carrying an Rh-positive baby, your immune system can recognize the baby’s Rh protein as foreign. Small amounts of the baby’s blood sometimes cross the placenta into your bloodstream, particularly during delivery, miscarriage, or certain procedures. When that happens, your body may produce antibodies against the Rh protein.
This process is called sensitization, and it typically doesn’t cause problems during a first pregnancy because it takes time for the immune response to build. The real danger is in subsequent pregnancies. Once your body has made anti-Rh antibodies, they can cross the placenta and attack the red blood cells of a future Rh-positive baby. This condition, called Rh disease or hemolytic disease of the newborn, can cause severe anemia in the fetus, jaundice, and in the most serious cases, life-threatening complications.
The good news is that this is almost entirely preventable. Rh-negative mothers receive an injection of Rh immune globulin (commonly known by the brand name RhoGAM) at 26 to 28 weeks of pregnancy. If the baby turns out to be Rh-positive at delivery, a second dose is given within 72 hours after birth. The injection works by neutralizing any fetal Rh-positive blood cells that entered the mother’s bloodstream before her immune system has a chance to react to them. Since this treatment became standard, Rh disease has gone from a major cause of infant illness to something rarely seen.
Rh Factor and Blood Transfusions
Rh compatibility also matters when receiving blood. If you’re Rh-negative and receive Rh-positive blood, your immune system may produce antibodies against the Rh protein, similar to what happens in pregnancy. A first mismatched transfusion might not cause obvious problems, but once those antibodies are in place, a second exposure to Rh-positive blood can trigger a transfusion reaction. Your immune system attacks the donated red blood cells, destroying them in a process called hemolysis. Depending on severity, this can cause fever, kidney damage, or a medical emergency.
For this reason, Rh-negative patients are given Rh-negative blood whenever possible. Rh-positive patients can safely receive either Rh-positive or Rh-negative blood, since their immune system already recognizes the Rh protein as normal. This is also why O-negative blood is considered the universal donor type for emergencies: it lacks both ABO antigens and the Rh protein, so it’s least likely to trigger a reaction in any recipient.
Health Differences Linked to Rh Status
Beyond transfusions and pregnancy, researchers have looked at whether Rh-negative people differ in other health outcomes. A large cross-sectional study of over 3,100 people found that Rh-negative individuals reported higher rates of certain conditions, including allergies (especially skin allergies), digestive problems, heart issues, and mental health disorders like panic disorders and attention deficits. Rh-negative subjects also reported more frequent headaches, greater fatigue, and more visits to specialists in dermatology, psychiatry, and gynecology.
One particularly interesting finding involves the parasite that causes toxoplasmosis. Research on blood donors found that Rh-positive individuals, especially those who carry one positive and one negative gene copy, showed greater resistance to the effects of toxoplasmosis infection compared to Rh-negative individuals.
These associations are statistical patterns, not guarantees. Being Rh-negative doesn’t mean you’ll develop any of these conditions, and scientists don’t yet have a clear biological explanation for why Rh status would influence susceptibility to allergies or mental health conditions. The Rh protein’s exact role in the body is still being studied, and these correlations may eventually be explained by other genetic factors that tend to travel alongside the Rh gene.
How to Find Out Your Rh Type
Your Rh factor is determined through a simple blood test, the same one that identifies your ABO blood type. If you’ve ever donated blood, had surgery, or been pregnant, your Rh status is likely already in your medical records. Prenatal blood work routinely includes Rh typing at the first visit, since knowing a mother’s Rh status early is essential for deciding whether preventive treatment is needed.
If you’ve never been tested and are curious, any standard blood typing panel will include your Rh status. The result is straightforward: positive or negative, with no ambiguous middle ground. Unlike some blood markers that fluctuate, your Rh factor is set by your genes and stays the same for life.