The Er blood group system consists of five human red blood cell surface antigens, Era, Erb, Er3, Er4 and Er5.[1] The incidences of Era and Er3 are each greater than 99% of the human population, while the incidence of Erb is less than 0.01%.[2][3] Er4 and Er5 are found at a high frequency in the general population.[1]
Individuals with antibodies against Er3 may develop acute hemolytic transfusion reaction upon transfusion with an incompatible unit, while Era and Erb are unlikely to be clinically significant.[4] The clinical significance of antibodies against Er4 and Er5 is poorly understood due to a lack of data, but two cases of severe hemolytic disease of the fetus and newborn have been reported in women with these antibodies.[1] Expression of the Er blood group antigens is controlled by the gene PIEZO1.[1]
Era was first identified in 1982 and Erb was identified in 1988.[2] Er was recognized as a blood group collection by the International Society of Blood Transfusion in 1990[2] and promoted to a blood group in 2022.[1]