This report is part of a series reporting the proceedings from the International Society of Blood Transfusion (ISBT) Working Party on Immunohaematology Workshop on the Clinical Significance of Red Blood Cell Alloantibodies. The aim of the workshop was to review information regarding the clinical significance of alloantibodies to red blood cell antigens recognized by the ISBT. The first 12 systems will be covered in this report. It is understandable that many of the most clinically important antibodies are directed toward antigens found in the blood group systems discovered earlier in history. The ABO system was the first to be discovered and remains the most clinically important regarding transfusion.