Abstract
Background and Objectives
Having more than 50 antigens, the Rh system is one of the most polymorphic blood group systems where clinically it is the second important blood group system after ABO. E,C,D,e,c are the most frequent antigens of this system. Considering the few reports on the high frequency of these antigens in regular donors of  Isfahan Blood Center, the aim of the study was to determine their phenotype and frequency, identify people with rare antigens, and form the data bank.
 
Materials and Methods
The study was descriptive with 310 regular O type donors referring to Isfahan Blood Center in 2016 included by Nonprobability-Convenience sampling. A blood suspension of 3-5% in the normal saline 0.9% was prepared and exposed to Rh anti sera by direct agglutination. Then, the Rh phenotype was determined based on the most common genotype.
 
Results
Among all the observed reactions, the highest frequency was e (95.16%) and the frequency rates of other antigens were 87.1%, 71.29%, 74.19%, and 32.9% for D-, C-, c-, and E-, respectively. The most common phenotype was R1r (27.42%), while the lowest being r"r (0.32%), and R2RZ (0.32%).
 
Conclusions 
Generally, the research on phenotype determination & RBC antigen frequencies in regular blood donor population would lead to accessible compatible blood types for regular blood recipients in minimum time and to reduction in hemolytic- alloimmune reactions.