Reference — Hematology
ABO & Rh Blood Types Reference
One rule generates the entire compatibility system: you cannot give red cells carrying an antigen the recipient has antibodies against. Learn the antigen/antibody pairs and every “who can receive what” question answers itself.
Educational use only. Compatibility decisions belong to the blood bank — crossmatch and the unit tag are authoritative, never memory. This material supports nursing education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional policy, or medical direction. Always follow facility protocols and current provider orders.
The ABO System
| Blood Type | Antigens on RBCs | Antibodies in Plasma | Can Receive RBCs From |
|---|---|---|---|
| A | A antigen | Anti-B | A, O |
| B | B antigen | Anti-A | B, O |
| AB | A and B antigens | None | A, B, AB, O — universal recipient |
| O | None | Anti-A and Anti-B | O only — but universal RBC donor |
The antibodies do the damage: a type A patient carries anti-B, so type B or AB red cells trigger immediate hemolysis. Type O cells carry no A/B antigens — nothing for anyone’s antibodies to attack.
Plasma Reverses the Rules
Red cell compatibility is about the antigens in the bag; plasma compatibility is about the antibodies in the bag. Type O plasma is full of anti-A and anti-B — fine for an O recipient, dangerous for everyone else. Type AB plasma contains no ABO antibodies at all.
So the universal roles flip: O-negative is the universal RBC donor; AB is the universal plasma donor. AB patients are universal RBC recipients but can only receive AB plasma.
The Rh Factor
Rh-positive means the D antigen is present (~85% of people); Rh-negative means it is absent. Unlike ABO antibodies, anti-D is not naturally present — an Rh-negative person makes it only after exposure to Rh-positive blood (transfusion or pregnancy). That first exposure sensitizes; the next exposure hemolyzes.
Practice rules: Rh-negative patients receive Rh-negative red cells; Rh-positive patients can receive either. In emergencies, uncrossmatched O-negative is the default for women of childbearing potential precisely to avoid D sensitization.
RhoGAM (Rh immune globulin) applies the same logic in pregnancy: given to Rh-negative mothers (commonly at 28 weeks, after delivery of an Rh-positive infant, and after any fetal-maternal bleeding event) to prevent sensitization that would attack a future Rh-positive fetus.
Related Resources
Standards & sources
Fact-checked Jun 21, 2026This page is written to align with AABB (transfusion standards) · American Society of Hematology (ASH). It is an educational summary, not a citation of any single document — always verify specific doses, values, and protocols against current guidelines and your facility policy. How we source content →