The Er blood group system consists of five human red blood cell surface antigens, Era, Erb, Er3, Er4 and Er5.
[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.