ABO

Most important blood group system in transfusion medicine
- Fundamental in determining compatibility for red blood cell (RBC) transfusions.
Determines compatibility for RBC transfusions
- Essential for safety and efficacy in blood transfusions.
Naturally occurring antibodies present without prior exposure
- Individuals possess antibodies against ABO antigens that they do not express on their RBCs due to natural immunization.
Incompatible transfusion → severe hemolytic reaction
- Can lead to significant and dangerous hemolysis of transfused RBCs.

Located on red blood cell surface
- Antigens A and B are found attached to the RBCs.
Also found on tissues and in secretions (if secretor)
- ABO antigens may be present in bodily fluids if the individual is a secretor.
Derived from H antigen
- The precursor for A and B antigens. The H antigen is modified to produce specific ABO antigens.
A antigen:
- N-acetylgalactosamine is added to the H antigen.
B antigen:
- Galactose is added to the H antigen.
O group:
- No additional sugar added; the H antigen remains unchanged.

Naturally occurring (formed early in life)
- Individuals develop antibodies during the first few months of life, influenced by gut microbiota exposure.
Mainly IgM (can fix complement → intravascular hemolysis)
- IgM antibodies are highly effective at activating the complement system leading to hemolysis.
Blood Group Specific Antibodies:
- Group A → Anti-B
- Group B → Anti-A
- Group AB → No antibodies
- Group O → Anti-A and Anti-B

Blood Type | Antigen(s) | Antibody(ies) | Can Receive | Can Donate
- A | A | Anti-B | A, O | A, AB
- B | B | Anti-A | B, O | B, AB
- AB | A & B | None | All (universal recipient) | AB
- O | None | Anti-A & Anti-B | O only | All (universal donor)

Patient RBCs tested with anti-A and anti-B reagents
- The sample is mixed with anti-A and anti-B antibodies to identify antigens present on RBCs.
Detects presence of A or B antigens
- Result determined by hemagglutination.
Agglutination = antigen present

Patient serum tested with A and B reagent cells
- The patient serum is mixed with A and B RBCs to test for antibodies.
Detects antibodies in plasma
- Evaluates the presence of anti-A and anti-B antibodies in the patient's serum.
Agglutination = antibody present

Forward and reverse typing must agree
- Consistency in results is critical for accurate blood typing.
Example Interpretations:
- Agglutination with Anti-A → Type A
- Agglutination with Anti-B → Type B
- Both → Type AB
- Neither → Type O

Weak/missing antibodies (especially in elderly and newborns)
- May show inconsistent results due to low antibody levels.
Weak antigens (subgroups of A)
- Can lead to unexpected results and confusion in typing.
Unexpected antibodies
- Presence of other antibodies not anticipated could complicate typing.
Technical errors
- Mistakes such as using wrong cells or contamination can lead to erroneous results.
Cold agglutinins
- Some patients produce antibodies that react at lower temperatures causing discrepancies.

Most dangerous transfusion reactions
- ABO incompatibility is a leading cause of severe transfusion reactions.
IgM antibodies activate complement
- This activation can lead to rapid destruction of transfused RBCs through complement-mediated lysis.
Causes intravascular hemolysis
- The rupture of RBCs in circulation leading to hemoglobin release into plasma.
Symptoms:
- Classic symptoms of hemolytic reactions include fever, chills, hemoglobinuria, and shock per acute hemolysis.

Always check forward AND reverse typing
- Ensure both tests align for accurate blood typing.
O = universal donor (RBCs)
- O group can donate to any blood type.
AB = universal recipient
- AB group can receive RBCs from any blood type.
ABO mismatch = most severe reaction
- This mismatch poses the highest risk of hemolytic transfusion reactions.
IgM = complement activation
- Understanding the role of IgM in activating the complement system is critical in predicting transfusion outcomes.