Blood Bank / Immunohematology
What to master
Blood bank questions reward system-level reasoning: you predict what an antibody will do in vivo, then pick the safest product under time pressure. Know ABO genetics and the reciprocal isohemagglutinins of Landsteiner's rule, why anti-A and anti-B are IgM and complement-activating, and why anti-D and most other significant antibodies are IgG that react at 37 °C and in the indirect antiglobulin test. Learn the logic of the antibody screen, the panel, the autocontrol, the direct antiglobulin test, elution, and adsorption. Separate clinically significant systems (Rh, Kell, Duffy, Kidd, MNS, Diego) from those usually not significant (Lewis, P1, I, Lutheran, M/N). Understand enhancement: LISS and PEG accelerate the IAT; proteolytic enzymes destroy Duffy and MNS antigens but enhance Rh and Kidd, and Kidd antigens show dosage.
Traps
Expect ABO discrepancies (A2 with anti-A1, acquired B, cold autoanti-I, absent isohemagglutinins in infants or immunocompromised patients), weak and partial D, and the request for least incompatible blood, which requires a physician order and documented monitoring. Do not use weak D testing on cord bloods to decide RhIG, and remember that passive anti-D after RhIG can mimic alloimmunization.
Worked example
A Kleihauer-Betke stain shows 1.2 % fetal cells in an estimated 5000 mL maternal blood volume: 1.2 % of 5000 = 60 mL of fetal red cells. At 15 µg of RhIG per mL of fetal red cells, 60 × 15 = 900 µg, so 3 vials of 300 µg are needed.