What observation is apparent with one of the antibodies present on a panel showing reactivity?

The correct choice indicates that one antibody is only reacting with homozygous cells. This observation is significant in immunohematology as it suggests that the antibody in question is demonstrating a dosage effect. Dosage means that the expression of specific antigens varies depending on whether the individual cells are homozygous (two identical alleles for the antigen) or heterozygous (two different alleles).

In this case, if the antibody only reacts with homozygous cells and not with heterozygous ones, it indicates that the antibody is likely targeting an antigen where the homozygous state has a stronger expression compared to the heterozygous state. This is a common scenario with certain blood group systems where antigens increase in strength when present in greater quantities, which occurs in homozygous individuals.

This observation can be particularly helpful in blood bank practices for identification of antibodies. While other options discuss various reactivities involving heterozygous cells or the presence of more than one antibody, they do not specifically illustrate the unique reaction pattern associated with homozygosity and its implications for dosage effect. The presence of only one antibody reacting with homozygous cells highlights the specificity and strength of that antibody towards antigens in a homozygous context