The blood type AB negative (AB-) is a specific and relatively uncommon blood classification determined by markers on red blood cells. Receiving a blood transfusion is a precise medical procedure because the immune system must recognize the donated blood as safe. Compatibility is determined by matching these cellular markers, which prevents a life-threatening immune reaction. Understanding which blood types an AB- individual can receive requires looking at the two most significant systems: the ABO and the Rh factor.
The Foundation: ABO and Rh Systems
Blood type is categorized primarily by the ABO system, based on the presence of A and B antigens on the surface of red blood cells (RBCs). Antigens are molecules that can trigger an immune response if they are foreign to the body. An individual with AB blood has both A and B antigens on their RBCs, unlike Type O, which has neither.
Because AB- individuals possess both A and B antigens, their plasma does not contain antibodies against either A or B antigens. The second component is the Rh system, which determines the positive or negative designation. The “negative” sign in AB- indicates the absence of the D antigen, the most significant component of the Rh factor.
The absence of the D antigen means the AB- individual’s immune system will recognize the D antigen as foreign. If they receive D-positive blood, their body will produce anti-D antibodies, which would attack the transfused cells. Therefore, safe red blood cell transfusions for AB- patients must exclude any blood that contains the D antigen, which is the defining characteristic of all “positive” blood types.
Which Red Blood Cells AB- Can Safely Receive
The unique combination of having both A and B antigens but lacking the Rh factor dictates compatibility for red blood cell transfusions. Since the AB- immune system has no anti-A or anti-B antibodies, it will not attack A, B, or AB antigens. The only restriction is the Rh factor, meaning the transfused red blood cells must be Rh-negative.
This immunological profile allows an AB- recipient to safely receive red blood cells from any of the four Rh-negative blood types. These compatible types are AB negative, A negative, B negative, and O negative. The donated RBCs must not contain any antigens that the recipient’s plasma could attack.
All four of these acceptable blood types lack the D antigen, satisfying the Rh-negative requirement. The AB- recipient can accept A- and B- blood because their immune system lacks the corresponding anti-A and anti-B antibodies. The O- blood type, which lacks all A, B, and D antigens, is also fully compatible with AB- patients.
AB- Blood: Rarity and Plasma Donation
The AB- blood type is one of the rarest, found in less than 1% of the population. This scarcity makes AB- donations particularly valuable to the blood supply system. Despite the limited options for receiving red blood cells, the AB- blood type holds a special status in plasma donation.
Plasma is the liquid component of blood that carries antibodies, clotting factors, and proteins. Since AB- individuals have both A and B antigens on their cells, their plasma contains neither anti-A nor anti-B antibodies. This lack of antibodies means that AB- plasma can be safely transfused into patients of any ABO blood type without causing a reaction.
Due to this universal compatibility, AB- donors are often called “universal plasma donors.” The rules for receiving red blood cells are specific, but the rules for donating plasma are universal. This allows AB- blood products to serve a broad range of patients in need.
Emergency Transfusion Protocols
In a critical emergency, a patient may require an immediate blood transfusion before their specific blood type can be fully identified. These situations demand the use of a universal donor product safe for nearly everyone. For red blood cell transfusions, the O- blood type is considered the universal donor.
O- red blood cells lack all A, B, and D antigens, making them safe to give to patients of any blood type, including AB-. When an AB- patient is bleeding severely and their blood type is unknown, O- blood is administered immediately to stabilize them. Once the patient’s blood type is confirmed to be AB-, the medical team switches the transfusion to one of the four Rh-negative compatible types.
This protocol ensures a patient receives the safest possible product immediately, bridging the time until type-specific blood is available. While O- is the default for unknown-type emergencies, the goal is to transition to AB-, A-, B-, or O- as soon as possible to conserve the limited O- supply.