Blood type is a way to describe which marker molecules are found on the surface of a person’s red blood cells. These markers matter because the immune system can recognize unfamiliar blood cells as foreign and attack them. Safe blood transfusion depends on matching donor blood to the recipient’s antibodies.
A mismatch can cause clumping of red blood cells, blocked vessels, kidney damage, shock, or death.
The two most important blood group systems for transfusion are ABO and Rh. In the ABO system, red blood cells may carry A antigens, B antigens, both, or neither, while the plasma contains antibodies against the missing ABO antigens. In the Rh system, the D antigen is the key marker, and people who have it are Rh positive while people who lack it are Rh negative.
Compatibility rules follow from antigen and antibody interactions, so type O negative red blood cells are often called universal donor cells and type AB positive patients are often called universal recipients for red blood cell transfusions.
Key Facts
- Type A blood has A antigens on red blood cells and anti-B antibodies in plasma.
- Type B blood has B antigens on red blood cells and anti-A antibodies in plasma.
- Type AB blood has A and B antigens and usually has no anti-A or anti-B antibodies.
- Type O blood has no A or B antigens and has both anti-A and anti-B antibodies in plasma.
- Rh positive means the D antigen is present; Rh negative means the D antigen is absent.
- Agglutination occurs when antibodies bind matching antigens on red blood cells, forming clumps.
Vocabulary
- Antigen
- A cell-surface molecule that can be recognized by the immune system.
- Antibody
- A Y-shaped immune protein that binds to a specific antigen.
- Agglutination
- The clumping of red blood cells caused by antibodies binding to antigens on those cells.
- ABO blood group
- A blood classification system based on whether red blood cells have A antigens, B antigens, both, or neither.
- Rh factor
- A blood classification based mainly on the presence or absence of the D antigen on red blood cells.
Common Mistakes to Avoid
- Confusing antigens with antibodies: antigens are usually on red blood cells, while antibodies are in the plasma and attack matching foreign antigens.
- Assuming O positive is the universal donor: O positive red blood cells still have the Rh D antigen, so they may be unsafe for Rh negative recipients.
- Forgetting that ABO compatibility for red blood cells is different from plasma compatibility: plasma transfusions depend on the donor’s antibodies, not just the donor’s red blood cell antigens.
- Thinking Rh negative blood always has anti-D antibodies: Rh negative people usually make anti-D antibodies only after exposure to Rh positive blood, such as through transfusion or pregnancy.
Practice Questions
- 1 A patient has type A positive blood. Which red blood cell types are generally compatible for transfusion into this patient: A+, A-, B+, B-, AB+, AB-, O+, or O-?
- 2 In a class of 200 students, 42% have type O blood and 15% are Rh negative. If Rh status and ABO type are treated as independent, about how many students would be expected to have type O negative blood?
- 3 A sample of red blood cells clumps when mixed with anti-A serum, does not clump with anti-B serum, and clumps with anti-D serum. What is the blood type?
- 4 Explain why a type AB positive person can receive red blood cells from many blood types, but a type O negative person must receive a much narrower range of red blood cell types.