What may cause a positive reaction with anti-B while being negative for A1 cells?

A positive reaction with anti-B while being negative for A1 cells can be explained through the concept of a weak A subgroup. In individuals with weak A subgroups, the A antigen expression is diminished, resulting in a weaker reaction or no reaction when A1 cells are tested with anti-A reagents. However, these individuals can still express the B antigen normally.

In this context, the presence of anti-B antibodies will lead to a positive reaction when testing B antigens, while the weakly expressed A antigen does not elicit a strong enough reaction to register positive with A1 cells. This scenario illustrates how variations in blood group antigens can produce complex serological results that may not align with typical expectations in the ABO blood typing system.

The other options, while relevant to blood group serology, do not specifically account for the combination of a positive result for anti-B and a negative result for A1 cells in the same manner as weak A subgroup does.