Characteristics of the main blood groups in SCD patients and their identification. This figure shows the 5 blood group systems most frequently involved in alloimmunization. The following are shown for each blood group: blue, the common antigens rarely expressed; green, the partial antigens; black, the low-frequency antigens most frequently expressed; red, the high-frequency antigens most frequently absent (rare blood groups). They can be identified by serology and genotyping or genotyping alone.