An autoanti-Kp b immunoglobulin M that simulates antigen suppression.

Background: Patients may present with an antibody against a blood group antigen, a negative direct antiglobulin test (DAT), and a null phenotype. Typically, this represents an alloantibody in a null individual. However, on occasion, the antibody disappears coincident with conversion to a positive red blood cell (RBC) phenotype. This has been called antigen loss, antigen suppression, or weakened antigenicity. Herein, a unique serologic profile that mimics this pattern, when in fact antigen loss did not occur, is described.

Study Design And Methods: RBCs and serum were analyzed using a gel microtyping system and flow cytometry. Genomic DNA was amplified by polymerase chain reaction and sequenced.

Results: Initially, an anti-Kp(b) was detected in MTS gel, RBCs typed K-k-Kp(b-), and the DAT was negative for immunoglobulin G (IgG). Later, the anti-Kp(b) disappeared and RBCs phenotyped as K-k+Kp(b+). Analysis of initial specimens by flow cytometry identified an immunoglobulin M (IgM) anti-Kp(b) with a positive IgM-specific DAT; eluates contained an anti-Kp(b) at immediate spin. Supporting the presence of the Kell glycoprotein, RBCs agglutinated with anti-Js(b). Sequencing showed homozygosity for Kp(b) with no mutations surrounding the Kp(b) polymorphism.

Conclusion: In antigen loss, antibody masking is excluded by a negative DAT. However, because typical DAT reagent does not detect IgM, such reasoning was inaccurate in the current case. In addition, an anti-Kp(b) resulted in RBCs typing k-, even though no anti-k was detected. Overall, this case suggests that an IgM may mask adjacent epitopes and illustrates the potential to mistake a non-IgG autoantibody as antigen loss.