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PA21.1-6 | ABO & Rh Blood Group Systems — Part 1
CLINICAL SCENARIO
A 28-year-old woman receives her first red-cell transfusion after a road-traffic accident. Within 10 minutes she develops fever, rigors, flank pain, and her urine turns dark red. Her haemoglobin drops further rather than rising. The blood bank phones: the unit was ABO-incompatible. Why does an error of one antigen letter cause catastrophic intravascular haemolysis in minutes — while an Rh mistake may cause no symptoms at all on first exposure? The answer lies in two fundamentally different immune mechanisms that this module will unpack.
WHY THIS MATTERS
Blood group knowledge underpins every transfusion, obstetric booking, and organ-transplant work-up you will perform. ABO and Rh incompatibility are the two leading causes of preventable transfusion-related death. Understanding the immunology — IgM vs. IgG, natural vs. immune antibody, complement activation vs. placental crossing — directly determines how you investigate a transfusion reaction, counsel a pregnant Rh-negative woman, and explain anti-D prophylaxis to a patient.
RECALL
Before starting, briefly reflect on:
• What is an antigen? What is an antibody? (Year-1 Physiology / Immunology)
• What is the difference between IgM and IgG? Which crosses the placenta?
• What does complement activation cause at the cell surface?
• What is agglutination? (You observed this principle in Year-1 practicals.)
You do NOT need prior transfusion knowledge — this module builds it from scratch.
RBC Surface Antigens: The Concept
Red blood cells carry hundreds of blood group antigens — oligosaccharide chains or proteins displayed on the outer leaflet of the cell membrane. These antigens are genetically determined and remain stable throughout life. They matter clinically because, if foreign, they can trigger an alloimmune response — the recipient's immune system produces antibodies against the donor's antigens, causing haemolysis.
Two systems dominate clinical practice:
1. ABO system — based on oligosaccharide antigens; associated with naturally occurring antibodies present even without prior exposure.
2. Rh system — based on a transmembrane protein (RhD polypeptide); associated with immune antibodies that form only after sensitisation.
The distinction between natural and immune antibodies is the key to understanding why these two systems behave so differently.
ABO Antigens: Biochemistry
ABO antigens are oligosaccharide chains attached to glycoproteins and glycolipids on the RBC surface. The foundation is the H substance (H antigen), a precursor oligosaccharide synthesised by the H gene product (fucosyltransferase), which adds a fucose residue to a core sugar chain.
The ABO gene at chromosome 9q34 encodes a second glycosyltransferase that modifies H substance:
• A allele → N-acetylgalactosaminyltransferase adds GalNAc → A antigen
• B allele → galactosyltransferase adds galactose → B antigen
• O allele → encodes a non-functional enzyme → H substance remains unconverted
| Blood Group | Antigen on RBC | Antibody in Plasma |
|---|---|---|
| A | A | Anti-B |
| B | B | Anti-A |
| AB | A + B | Neither |
| O | H only | Anti-A + Anti-B |
IMPORTANT — the antibody pattern in the right column is Landsteiner's Law (next block).
ABO Blood Group Biochemistry and Landsteiner's Law
Landsteiner's Law and Natural Isohaemagglutinins
Landsteiner's law states: the plasma always contains antibodies (agglutinins) against the ABO antigens absent from the person's own red cells.
These antibodies are called natural isohaemagglutinins (also: naturally occurring alloantibodies). Key properties:
• Immunoglobulin class: IgM (pentameric, does not cross the placenta)
• Present from approximately 3–6 months of age (stimulated by environmental antigens — gut flora, pollen, food — that share oligosaccharide structures with ABO antigens)
• Titre rises through childhood, remains high in adults
• Very efficient at activating complement → leads to immediate intravascular haemolysis when ABO-incompatible blood is transfused
Critical implication: a Group O recipient already has circulating anti-A and anti-B before ever receiving a transfusion. There is no need for prior exposure — this is why ABO incompatibility is immediately catastrophic on first exposure, unlike Rh incompatibility.
SELF-CHECK
A blood sample shows A antigens on RBCs and anti-B antibodies in plasma. According to Landsteiner's law, which blood group is this?
A. Group O
B. Group A
C. Group B
D. Group AB
Reveal Answer
Answer: B. Group A
Landsteiner's law: plasma contains antibodies against antigens NOT present on one's own RBCs. A antigens on RBCs → no anti-A in plasma; anti-B present because B antigen is absent. This is Group A. Group AB would have no antibodies; Group O would have both anti-A and anti-B; Group B would have A antigens absent and anti-A present.