Blood is categorized into eight main types based on the presence or absence of specific protein molecules, known as antigens, found on the surface of red blood cells. These classifications involve the ABO system (A, B, O, and AB) and the Rhesus (Rh) system (positive or negative). B Negative is a specific combination of these two major blood typing systems. This type is relatively uncommon, which has important implications for blood transfusions and pregnancy.
The Biological Basis of B Negative Blood
B negative blood combines features from the ABO and Rh systems. The “B” designation means the red blood cells carry the B antigen, and the plasma naturally produces antibodies against the A antigen.
The “Negative” part refers to the absence of the Rh D antigen on the surface of the red blood cells. If the Rh D antigen is absent, the blood is Rh negative. A person with B negative blood does not naturally have antibodies against the Rh D antigen, but their immune system will produce them if exposed to Rh positive blood.
The presence of the B antigen and the lack of the Rh D antigen are inherited. The body’s production of antibodies against missing antigens is a fundamental defense mechanism. These antibodies attack foreign antigens introduced during a transfusion, which necessitates blood type matching.
Compatibility in Blood Transfusions
Compatibility is determined by ensuring the recipient’s circulating antibodies do not destroy the donor’s red blood cells. For B negative recipients, safe transfusion rules are restrictive due to the presence of anti-A antibodies and the potential to develop anti-Rh antibodies. Consequently, B negative individuals can safely receive red blood cells only from B negative or O negative donors. O negative blood is known as the universal donor for red blood cells because it lacks the A, B, and Rh D antigens.
For donation purposes, B negative blood is highly valued because it can be given to all B and AB patients, regardless of their Rh status. A B negative donor’s red blood cells can be safely transfused into B negative, B positive, AB negative, and AB positive recipients. This is because B negative blood lacks both the A antigen and the Rh D antigen, avoiding immune reactions in all these groups.
The rarity of B negative blood, affecting less than 2% of the population globally, emphasizes the significance of B negative donors. Maintaining a sufficient supply is challenging for blood banks, especially since B negative recipients cannot safely receive the more common B positive blood.
B Negative Blood and Rh Incompatibility During Pregnancy
The Rh-negative status of B negative blood is a serious consideration during pregnancy if the mother is carrying an Rh-positive fetus, which occurs when the father is Rh positive. Although the mother and fetus’s blood systems are normally separate, fetal blood can enter the mother’s circulation during delivery, miscarriage, or certain procedures.
If an Rh-negative mother is exposed to Rh-positive red blood cells, Rh sensitization can occur. The mother’s immune system recognizes the Rh D antigen as foreign and begins producing anti-Rh antibodies. These antibodies usually do not affect the first Rh-positive baby, but they remain in the mother’s system.
In subsequent pregnancies with an Rh-positive baby, these anti-Rh antibodies can cross the placenta and attack the baby’s red blood cells, potentially causing severe anemia and other complications. Modern medical practice addresses this risk with an injection of Rh immune globulin (RhoGAM). RhoGAM contains antibodies that temporarily prevent the mother’s immune system from making its own anti-Rh antibodies. This shot is typically given around the 28th week of pregnancy and again shortly after delivery if the newborn is confirmed to be Rh positive.