Which genotype usually shows the strongest reaction with anti-D?

The strongest reaction with anti-D is typically seen with individuals who possess the D antigen in a homozygous state, which corresponds to the genotype D-/D- in this context. This genotype can produce a robust immune response because having two alleles for the D antigen means that there is an increased expression of the D antigen on the red blood cells.

When anti-D antibodies are introduced to the blood of an individual with the D-/D- genotype, there is a high likelihood of a strong serological reaction due to the presence of two identical alleles that express the D antigen. The homozygous state enhances the quantity of the antigen present, making it easier for the antibodies to latch onto and form immune complexes.

In contrast, other suggested genotypes may express D antigen in a less homozygous form, leading to weaker reactions. For example, generally heterozygous genotypes or those that are deficient in antigen expression do not elicit the same intensity of reaction with anti-D as the homozygous presence seen in the D-/D- genotype. Hence, the D-/D- combination is associated with the strongest reaction to anti-D antibodies in serological testing.