If anti-K reacts 3+ with a donor cell with genotype KK and 2+ with genotype Kk, what is this an example of?

This scenario demonstrates the phenomenon known as dosage effect. The anti-K antibody is reacting more strongly (3+) with the homozygous KK cells compared to the heterozygous Kk cells (2+). In immunology, dosage refers to the concept that the strength of an antibody reaction can vary depending on the number of antigenic sites present.

In this case, the donor cells with the KK genotype express two K antigens on their surface, leading to a stronger interaction with the anti-K antibody. Conversely, the Kk genotype expresses only one K antigen along with a non-K antigen, resulting in a weaker reaction. This illustrates how the amount of antigen present can influence the strength of the antibody response, aligning perfectly with the principle of dosage.

The incorrect alternatives encompass concepts that do not apply here. Linkage disequilibrium refers to the non-random association of alleles at different loci, which is not relevant in this antibody-antigen interaction context. Homozygosity and heterozygosity describe the genetic makeup concerning alleles (both alleles the same vs. different), but do not directly address the impact on antibody reactivity based on the number of identical antigen sites.