What is the most likely explanation for a patient presenting with R1R1 phenotype after being transfused with Rir red cells?

The patient presenting with the R1R1 phenotype after receiving Rir red cells suggests that the transfusion has not adequately altered the patient’s genotype to express the R1R1 antigen profile. The R1R1 phenotype indicates the presence of specific Rh antigens on the red blood cells.

When an R1R1 patient is transfused with Rir red cells, the Rir cells would primarily express the 'r' antigen due to the presence of the lowercase 'r' allele, which does not have the R1 antigen. However, transfusion in this scenario would not introduce the necessary Rh D antigens to change the patient's overall phenotypic expression to show R1R1 due to the predominance of their own antigenic expression. The recipient's immune system likely recognizes the transfused cells as foreign, but it does not alter the existing antigen expression of R1R1, leading to the patient still being phenotypically classified as R1R1.

The background provided by the question indicates that the transfused red cells do not change the patient's phenotype. Thus, this understanding confirms that the right answer is tied to the relationship between the patient’s genotype and the transfused red cells, leading to the conclusion that an R