Based on global sharing of blood products, a more universal language related to phenotypes was needed. The International Society of Blood Transfusion (ISBT) established a uniform nomenclature that is eye- and machine-readable and is in keeping with the genetic basis of blood group systems. In this terminology, a six-digit number is adopted for each authenticated antigen belonging to a blood group system where the first three numbers represent the system and the last three numbers represent the antigenic specificity. In addition, a given antigen may be written using an alphanumeric designation with the blood group system in uppercase letters followed by the number assigned to the antigenic specificity.
ISBT term for the five major Rh antigens:
D 004001 - RH1
C 004002 - RH2
E 004003 - RH3
c 004004 - RH4
e 004005 - RH5
Howard, PR, Basic & Applied Concepts of Blood Banking and Transfusion Practices, 4th ed, Elsevier, St. Louis, MO, 2017, p 129.
Quinley, ED, Immunohematology Principles and Practice, 3rd ed. Lippincott, Williams, and Wilkins, Philadelphia, PA, 2011, p 141.