A patient has anti-E, and crossmatching results in a delayed reaction days later. The patient is identified as R1R1 and crossmatched with a unit labeled D-C-E-c+e+. What happened?

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The key idea is that antibody screening can miss clinically significant alloantibodies, allowing an incompatible unit to be transfused and then causing a delayed reaction after the transfusion.

Here the patient has an anti-E antibody. The donor unit has the E antigen (E+). If the antibody screen failed to detect anti-E—perhaps because the screening cells used did not express E or the antibody titer was low—the lab wouldn’t withhold an E+ unit. Transfusing RBCs carrying the E antigen into someone with anti-E sets up an in vivo reaction that may not be evident immediately, but can develop days later as anti-E binds to the transfused cells and causes hemolysis. That’s why the crossmatch produced a reaction days after the transfusion: the anti-E wasn’t detected during initial screening.