Figure 5.

Epitopes targeted by HLA antibodies may be shared across multiple beads in a single assay, lowering the MFI detected on any individual bead. Multiple beads share the Bw6 epitope. In this case, the mismatched donor antigen of interest is B35, with a normalized bead MFI of 1216 (weak). Many centers would predict a negative FCXM with this donor; however, it was strongly positive. Closer examination reveals that the majority of antigens sharing the Bw6 epitope are clustered together in this MFI range. Neither a B7 nor a B5 cross–reactive epitope group explains this reactivity pattern entirely; rather, it is likely that an antibody to Bw6 is diluted across multiple beads sharing this epitope. During in vitro XM, where only the B35 target was present, the antibody can bind without epitope dilution, yielding a positive result. CREG, cross reactive group.