Hematology
Blood Types
ABO and Rh — surface antigens that determine compatible transfusion
Blood type is determined by surface antigens on red blood cells. The ABO system has four types (A, B, AB, O) based on which carbohydrate antigens are present. The Rh system adds + or − based on the D antigen. Type O− is the universal red cell donor; AB+ is the universal recipient. Mismatched transfusion triggers acute hemolytic reaction — preformed IgM antibodies cause intravascular hemolysis, DIC, renal failure, death. ABO incompatibility kills within minutes; even 10 mL of wrong blood is dangerous.
- ABO antigensA, B, both, or neither (O)
- Rh antigenD (positive or negative)
- Universal RBC donorO negative
- Universal recipientAB positive
- Plasma rule reversedAB plasma to all; O plasma to O only
- Anti-A/BIgM, naturally occurring
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Why blood types matter
- Transfusion safety. ABO mismatch kills; mandatory pre-transfusion typing.
- Pregnancy. Rh− mothers need RhoGAM to prevent HDN.
- Organ transplantation. ABO compatibility required (with rare desensitization protocols).
- Mass casualty. O− kept on hand for uncrossmatched emergency release.
- Forensics and paternity. Historical use; now superseded by DNA.
- Disease association. Type O lower risk of cardiovascular disease; type A higher gastric cancer; type O more severe cholera.
- Plasma exchange. Therapeutic apheresis requires type-specific replacement.
Common misconceptions
- "O is universal — give to anyone." Only O− red cells; O+ unsafe for Rh− women of childbearing age.
- Rh+ father guarantees Rh+ baby. If father is heterozygous, baby may be Rh−.
- Type AB blood is rare so it's most precious. AB plasma is precious; AB red cells are easy to match (any donor works).
- Blood types determine personality. Japanese folk belief; no scientific basis.
- One typing lasts forever. Re-type every admission; misidentification is the leading cause of fatal reactions.
- Self-donation always safe. Autologous units still risk clerical error and bacterial contamination.
Frequently asked questions
How does ABO work?
A gene encodes a glycosyltransferase. A allele adds N-acetylgalactosamine to the H antigen; B allele adds galactose; O allele is nonfunctional. Type A has A antigen, anti-B antibody. Type B: B antigen, anti-A. Type AB: both antigens, no antibodies. Type O: no antigens, both antibodies. Antibodies form in infancy from gut bacteria exposure.
Why is Rh negative dangerous in pregnancy?
An Rh− mother carrying an Rh+ fetus can develop anti-D antibodies after delivery, miscarriage, or trauma. Subsequent Rh+ pregnancies trigger hemolytic disease of the newborn (HDN) — IgG crosses placenta, attacks fetal RBCs, causing hydrops fetalis, kernicterus, stillbirth. Prevention: Rho(D) immunoglobulin (RhoGAM) at 28 weeks and within 72 hours of delivery.
How does cross-matching work?
Type and screen: ABO/Rh typing plus antibody screen. Major crossmatch: donor red cells + recipient plasma; checks for recipient antibodies against donor. Electronic crossmatch suffices if antibody screen is negative. In emergencies, give O− packed cells uncrossmatched. Each unit transfused requires patient ID double-check.
What happens in a transfusion reaction?
Acute hemolytic (ABO mismatch): fever, chills, flank pain, hemoglobinuria, hypotension, DIC, death. Mortality ~10%. Stop transfusion, save bag, support BP, force diuresis. Other reactions: febrile non-hemolytic (cytokines from leukocytes); allergic (IgA deficiency); TRALI (donor antibodies attack recipient leukocytes — leading cause of transfusion death).
Why is plasma rule reversed?
Plasma carries antibodies, not antigens. Type AB plasma has no anti-A or anti-B — safe for all. Type O plasma has both antibodies — only safe for O recipients. Same logic for platelets and FFP. Memorize: cells follow donor antigens; plasma follows donor antibodies.
Are there other blood groups?
Yes — over 40 systems and 600+ antigens. Clinically important: Kell, Duffy, Kidd, MNS, Lewis. Some patients develop alloantibodies after pregnancy or transfusion; future units must be antigen-negative. Sickle cell patients on chronic transfusion are typed for extended phenotype to prevent alloimmunization.
Who discovered blood types?
Karl Landsteiner, 1900 — Nobel Prize 1930. He mixed serum and red cells from colleagues, noticed agglutination patterns. Before this, transfusion was lethal lottery. Discovery enabled safe transfusion, which revolutionized surgery and trauma care. Rh discovered 1940 by Landsteiner and Wiener.