When the red cells of an individual fail to react with anti-U, they usually fail to react with which of the following?

The correct answer is that individuals whose red cells do not react with anti-U typically also do not react with anti-S. This relationship is rooted in the genetic variations associated with the MNS blood group system, wherein anti-U is specifically directed against a high-prevalence antigen, which is found in the S and s antigens.

In general, the absence of the U antigen is frequently associated with the presence of the S phenotype, indicating a non-reactivity with anti-S. This means individuals who lack the U antigen typically also lack the S antigen, leading to the failure to react with anti-S.

The connection between anti-U and anti-S is significant because it highlights the genetic link between the different antigens in the MNS blood group system. Understanding these antigen relationships is crucial in blood banking and transfusion medicine, especially in ensuring compatibility between donors and recipients.

While other options may relate to different blood group systems or antigens, they do not have the same direct relationship with the results of anti-U testing as anti-S does. Thus, individuals whose cells fail to react to anti-U will most consistently also fail to react to anti-S.