Which combination of T- and B-cell flow cytometric crossmatch results would be expected in a patient with anti-B35 antibodies?

• A. T cell negative, B cell negative
• B. T cell negative, B cell positive
• C. T cell positive, B cell positive
• D. T cell positive, B cell negative

Answer: (C)

In flow cytometric crossmatching, antibodies in the recipient’s serum against donor HLA antigens bind to donor lymphocytes and are detected by their fluorescence. HLA class I antigens, like HLA-B35, are present on almost all nucleated cells, including both T cells and B cells. So if the recipient has antibodies against B35 (a class I antigen), these antibodies will target and bind to both T cells and B cells in the donor sample, producing positive signals in both the T-cell and B-cell assays. Antibodies against HLA class II would bind to B cells (which express class II) but not to T cells, leading to a positive B-cell crossmatch with a negative T-cell crossmatch. Since anti-B35 is class I, the expected result is that both T cells and B cells show positive crossmatches.