Describe the course of alloimmunization to red cell antigens.

Primary alloimmunization becomes apparent by detection of antibody weeks or months after transfusion.  Approximately 1 - 3% of  transfused recipients develop detectable antibodies; the incidence increases in multi-transfused patients.  Hemolysis does usually not occur with 1o immunization because donor red cells  have disappeared before circulating antibody achieves a clinically significant level.