Blood Bank / Immunohematology
ABO and Rh fundamentals
Forward (cell) grouping detects RBC antigens with reagent antisera; reverse (serum) grouping detects plasma antibodies using A1 and B reagent cells. Adult forward and reverse results must agree before transfusion, and discrepancies are resolved rather than ignored. Infants under about 4 months have unreliable reverse grouping, so interpret forward typing only and rely on the maternal antibody history.
Traps that appear in questions
- Group A2 and A2B patients may produce anti-A1. It usually causes no hemolysis but creates a reverse-grouping discrepancy: serum reacts with A1 cells and not with A2 cells.
- Rouleaux mimics agglutination and disperses with saline replacement; cold autoantibody reactivity fades at 37 °C or after dithiothreitol treatment.
- Weak D: as a recipient, a weak D patient is managed as Rh-positive; donor units with weak D are labeled Rh-positive. Rh-negative women of childbearing age receive RhIG after fetomaternal hemorrhage, quantified by Kleihauer-Betke or flow cytometry.
- A positive antibody screen requires panel identification; a crossmatch-incompatible unit may still be safe if it lacks the corresponding antigen.
- In an emergency, group O red cells (Rh-negative for women of childbearing age) are issued until typing is complete.
Worked example
Forward group A, reverse showing 2+ agglutination with A1 cells and no reaction with B cells. Suspect A2 with anti-A1. Test the patient's cells with Dolichos biflorus lectin: A1 cells agglutinate, A2 cells do not. A negative result confirms A2, and A red cells may be transfused if needed.