Protease treatment of HOD × KEL RBCs removes HEL but not KEL antigen. (A) HOD × KEL RBC were treated with various concentrations of protease following incubation with PBS control, anti-KEL, or anti-HEL antibodies in vitro. The level of detectable KEL antigen (B), HEL antigen (C), or Ter119 antigen (D) was determined by flow cytometry. Means ± SD shown. Data are reflective of 2 independent experiments.