A sample from a 20-year-old donor shows anti-A 3+, anti-B 0, A1 cell 1+, B cell 3+, O cell 0. The most likely cause of this discrepancy is

A sample from a 20-year-old donor shows anti-A 3+, anti-B 0, A1 cell 1+, B cell 3+, O cell 0. The most likely cause of this discrepancy is

• A. Subgroup of A
• B. Anti-A1 antibodies
• C. Bombay phenotype
• D. Mixed-field reaction

Answer: (A)

When forward and reverse typing don’t line up, look for situations where A antigen is present but its expression is atypical. A subgroup of A, such as A2, often shows this pattern: the red cells still test positive with anti-A reagents (so you get an apparent type A in forward typing), but the antibody repertoire in the serum can include anti-A1 antibodies. Those anti-A1 antibodies react with A1 cells used in reverse typing, producing a small positive reaction with A1 cells, while the pattern you see with other cells (like B cells and O cells) follows the usual expectations for an A subgroup. Bombay would present no A or B antigens forward and anti-H in serum, which isn’t described here, and a mixed-field pattern would show two cell populations rather than the single-subgroup–driven oddity described. Thus, the discrepancy is best explained by a subgroup of A.

When forward and reverse typing don’t line up, look for situations where A antigen is present but its expression is atypical. A subgroup of A, such as A2, often shows this pattern: the red cells still test positive with anti-A reagents (so you get an apparent type A in forward typing), but the antibody repertoire in the serum can include anti-A1 antibodies. Those anti-A1 antibodies react with A1 cells used in reverse typing, producing a small positive reaction with A1 cells, while the pattern you see with other cells (like B cells and O cells) follows the usual expectations for an A subgroup. Bombay would present no A or B antigens forward and anti-H in serum, which isn’t described here, and a mixed-field pattern would show two cell populations rather than the single-subgroup–driven oddity described. Thus, the discrepancy is best explained by a subgroup of A.