Commonly but imprecisely called “Kell,” this is the most important antigen in the Kell blood group system. This antigen also goes by the name “KEL1.” While K is a relatively low-frequency antigen (present in only about 9% of Caucasians and 2% of African-Americans), it is very important for several reasons. First, it is highly immunogenic, meaning that a person who is negative for the antigen and is exposed to it (typically through either pregnancy or transfusion) is quite likely to form an antibody (more so that any other non-ABO antigen except D). Second, antibodies against the K antigen (typically IgG) are very capable of causing acute and delayed hemolytic transfusion reactions (HTR) as well as a unique form of hemolytic disease of the fetus/newborn (HDFN). Anemia associated with anti-K-induced HDFN can be particularly severe not so much because of destruction of K-positive circulating fetal red cells, but because the antibody suppresses erythropoiesis by attacking immature K-positive red cell precursors in the bone marrow. For more on the Kell Blood Group System, see the “Kell Kills” video.
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