There are many ways of grouping red blood cells based on their differences in antigens (the sugars or proteins that coats the red blood cell surface).

The body uses the cell surface antigens as identification markers which helps in separating the self from potentially harmful Invaders.

The most famous blood group systems are the ABO and Rhesus blood group systems. Meanwhile, many blood classification systems have been identified yearly by researchers. Although these blood types are very rare, they could be life-saving in certain situations.

The Er antigen, first discovered in 1988, was the 44th blood group system discovered. It is unusual but could be important in diagnosing a patient with a difficult case. Multiple generic mutations are associated with the Er blood group system. The Er blood group system consists of five (5) Er antigens: Era, Erb, Er3, Er4 and Er5. The incidences of Era and Er3 are each greater than 99% of the human population, whereas the incidence of Erb is less than 0.01%. The other antigens are very rare.

The gene that controls the expression of Er blood group antigens is PIEZO1. Piezo proteins are mechanosensory proteins that are used by the red cell to sense when it is being squeezed. The 5 Er antigens are 5 possible variations of Piezol on the surface of red blood cells that can lead to incompatibility.
Upon transfusion with an incompatible blood unit, individuals with antibodies against Er3 may develop acute hemolytic transfusion reaction, while Era and Erb may not be clinically significant.

The Er blood group system improves the possibility of accurately identifying and treating cases of blood incompatibility between mothers and their infants by giving infants blood transfusion in the womb.

References:

en.m.wikipedia.org

www.interestingengineering.com

www.sciencealert.com