Blood Banking: ABO/Rh, Antibody Screens, and Crossmatch Logic
Core concepts
Know the ABO system genetics and serology: group A has A antigen and anti-B; group B has B antigen and anti-A; group AB has both antigens and no antibodies; group O has neither antigen and both antibodies. Understand the two-sided principle - forward (cell) typing and reverse (serum) typing must agree; a discrepancy (e.g., weak subgroup, acquired B, or a missing antibody in infants) demands investigation. Rh typing focuses on the D antigen; weak D testing and the partial D concept matter for donor vs patient interpretation.
Antibody detection and compatibility
Antibody screens use reagent cells (usually two or three) with known antigen profiles; a positive screen triggers identification panels and, if clinically significant, antigen-negative crossmatched units. Know the indirect antiglobulin test (IAT) - antibody binds red cells in vitro, then anti-human globulin bridges to detect sensitization. Immediate-spin crossmatch detects ABO incompatibility; the AHG crossmatch detects other clinically significant antibodies. Know that clinically significant antibodies (Rh, Kell, Duffy, Kidd) cause hemolytic transfusion reactions or HDFN; IgM antibodies (anti-A, anti-B) fix complement and cause intravascular hemolysis.
Traps
Questions test the difference between a positive antibody screen and an incompatible crossmatch, and the emergency-release O-negative unit. Another trap: confusing forward and reverse typing directions, or assuming a positive DAT means a transfusion reaction (it can reflect autoantibodies or drug-induced hemolysis).
Worked example
A patient types as group A forward but reverse shows anti-A and anti-B. The discrepancy suggests a subgroup of A or an antibody problem; do not transfuse A cells until resolved. Rule of thumb: both sides must agree before release.