When typing ABO blood group, there was a discrepancy between forward and reverse methods: In the forward method: Red blood cell (RBC) showed strong agglutination with anti-A and anti-AB, and also showed weak agglutination with anti-B; In the reverse method: the sera showed agglutination with B red cells and no agglutination with A red cells. Tests were performed to conclude that the blood donor had group A and acquired B antigens, including: Auto control: Negative; Direct Coombs Test: Negative; RBCs showed 3+ agglutination with anti-A1; Antibody adsorption and elution (with anti-B): The eluted solution showed 3+ agglutination with B red cells; Donor's RBC did not react with anti-B at pH 5.0 but react with anti-B at pH 7.0.