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.


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.


CONCLUSIONS
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.
Bosco
Annmarie
A
Division of Hematology, Department of Medicine, University of Western Ontario, London, Ontario, Canada.
Xenocostas
Anargyros
A
Kinney
Jeff
J
Cadwell
Chantel M
CM
Zimring
James C
JC
D000372:Agglutination Tests; D000941:Antigens / Q000276:immunology; D001323:Autoantibodies / Q000032:analysis / Q000097:blood / Q000502:physiology*; D004252:DNA Mutational Analysis; D004912:Erythrocytes / Q000276:immunology; D006801:Humans; D007108:Immune Tolerance / Q000276:immunology*; D007075:Immunoglobulin M / Q000502:physiology*; D007626:Kell Blood-Group System / Q000235:genetics / Q000276:immunology*; D008297:Male; D008875:Middle Aged
D000941:Antigens; D001323:Autoantibodies; D007075:Immunoglobulin M; D007626:Kell Blood-Group System