Blood transfusions require precise matching of donor and recipient blood, as incompatible blood can cause a potentially life-threatening immune reaction. A correct understanding of blood type is fundamental, especially the Rhesus (Rh) factor, which determines if blood is positive or negative. For individuals with a negative blood type, the rules for compatible donors are precise and restrictive, designed to prevent a severe immune reaction.
Understanding Blood Type Basics
Blood typing relies on two classification systems: the ABO system and the Rh system. Blood type is determined by the presence or absence of specific protein markers, called antigens, on the surface of red blood cells. The ABO system categorizes blood into four groups—A, B, AB, and O—based on which antigens are present.
The body develops antibodies against any ABO antigens that are not present on its own red blood cells. For example, a person with Type A blood produces anti-B antibodies, which attack Type B red blood cells if transfused. This immune response, known as a hemolytic reaction, causes the destruction of the donor’s blood cells and can lead to organ failure.
The Rh system indicates whether the RhD antigen is present. If this protein is present, the person is Rh-positive; if absent, they are Rh-negative. Rh-negative individuals do not automatically have anti-Rh antibodies, but they rapidly develop them if exposed to Rh-positive blood. Therefore, any negative blood type recipient must receive only Rh-negative blood to prevent sensitization and a severe reaction.
Compatibility for O Negative Recipients
The O-negative (O-) blood type is the most restrictive in transfusion medicine. An O- recipient lacks the A, B, and RhD antigens. Because their body recognizes any of these markers as foreign, they can only safely receive red blood cells from O- donors. Receiving A-, B-, AB-, or any Rh-positive blood triggers a dangerous immune system attack.
This limitation makes O- recipients the most restricted group. Conversely, the absence of all antigens makes O- blood valuable. O- red blood cells are the universal donor, meaning they can be given to patients of any ABO or Rh type, as there are no antigens present for the recipient’s immune system to attack. This is useful in emergency trauma situations when a patient’s blood type is unknown and immediate transfusion is necessary.
Compatibility for Other Negative Types
While O- recipients are the most limited, the other negative blood types—A Negative (A-), B Negative (B-), and AB Negative (AB-)—have broader compatibility, but they all require Rh-negative blood. The Rh-negative status remains the primary constraint for all negative recipients.
An A- recipient can safely receive red blood cells from A- donors and O- donors. O- blood lacks the A antigen, which the A- recipient’s immune system would otherwise attack, and both are Rh-negative. Similarly, a B- recipient can be given blood from a B- donor or an O- donor, following the principle of avoiding the B antigen and all Rh-positive blood.
The AB- blood type is the most accommodating negative group, possessing both A and B antigens. An AB- recipient does not have antibodies against the A or B antigens, allowing them to receive blood from AB-, A-, B-, and O- donors. This makes AB- the universal recipient among all Rh-negative types, as they can accept red blood cells from any donor that is also Rh-negative.
The Rh Factor and Pregnancy
The Rh factor plays a major role in maternal and fetal health, extending beyond standard transfusions. If an Rh-negative woman becomes pregnant with an Rh-positive fetus, there is a risk of immune conflict. During childbirth or prenatal events, a small amount of the baby’s Rh-positive blood can enter the mother’s bloodstream.
The mother’s immune system recognizes the RhD antigen on the fetal red blood cells as foreign and begins creating anti-Rh antibodies, a process called Rh sensitization. The first pregnancy is usually unaffected because sensitization often happens late or at delivery, but the resulting antibodies remain in the mother’s system. In a subsequent pregnancy with another Rh-positive fetus, these pre-existing antibodies can cross the placenta and destroy the baby’s red blood cells, potentially causing severe anemia or hemolytic disease of the newborn.
To prevent sensitization, Rh-negative mothers are given an injection of Rh immune globulin, commonly known as the RhoGAM shot. This medication is administered around the 28th week of pregnancy and again shortly after delivery if the baby is confirmed Rh-positive. The Rh immune globulin temporarily prevents the mother’s body from producing permanent anti-Rh antibodies, safeguarding future Rh-positive pregnancies.