Immunopathology and Immunohematology (Blood Banking)
Core concepts
This domain spans immune mechanisms and transfusion medicine. Know hypersensitivity types (I–IV), autoantibody patterns (ANA, anti-dsDNA, anti-Smith), complement pathways, and the principles of immunoassays (ELISA, immunofluorescence, flow cytometry). In blood banking, master the ABO/Rh system, antibody screening and identification, the indirect antiglobulin test (IAT), and crossmatch procedures. Understand hemolytic transfusion reactions (acute vs delayed), the direct antiglobulin test (DAT) and its interpretation, and the workup of a positive antibody screen (panel cells, autocontrol, enzyme treatment). Know hemolytic disease of the fetus and newborn (HDFN) and Rh immunoglobulin prophylaxis. Recognize transfusion-related acute lung injury (TRALI) and transfusion-associated circulatory overload (TACO) as distinct entities.
Traps
A classic trap is confusing a positive DAT with a positive antibody screen: the DAT detects in vivo sensitization, while the IAT detects in vitro antibody. Another is misinterpreting an autocontrol-positive autocontrol suggests autoantibody or alloantibody to a high-incidence antigen. Questions may ask about ABO compatibility of plasma versus red cells (plasma must be ABO-compatible in the reverse direction). Do not forget that Rh-negative recipients should receive Rh-negative red cells, especially women of childbearing age.
Worked example / rule of thumb
For a positive antibody screen with a positive autocontrol, suspect an autoantibody or an alloantibody to a high-incidence antigen; perform an eluate and adsorption studies. Rule of thumb: red cells must be ABO-compatible with recipient plasma; plasma must be ABO-compatible with recipient red cells.