This excellently named term describes the occurrence of an unexpected (and seemingly impossible) antibody in an eluate; in other words, an antibody is present on someone’s red cells that really should not be there!
The best way to describe Matuhasi-Ogata is to give you an actual example: You are investigating a possible transfusion reaction, and the patient has a newly discovered anti-Fya. The DAT is positive, because the patient was recently transfused Fya-positive RBCs. When an eluate from these coated cells is analyzed, the anti-Fya is present, but an additional antibody, anti-E, is also present. At first glance, this makes no sense, because you discover in your workup that both the transfused cells and the patient are negative for the E antigen! The anti-E can be explained by the Matuhasi-Ogata phenomenon, which is likely “a nonspecific uptake of some IgG by red cells” in the presence of another antibody that is specifically targeted against antigens on those red cells (words in quotes from Issit and Anstee, Applied Blood Group Serology, 4th ed., pg 1132). In other words, in our scenario above, the anti-Fya has a good reason to be present on the Fya-positive red cells, but the anti-E is just nonspecifically along for the ride, because the RBCs lack E antigen! These antibodies are typically weakly reacting, and are usually easily explained.
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