This update of the Landsteiner-Wiener (LW) blood group system (Grandstaff Moulds MK. The LW blood group system: a review. Immunohematology 2011;27:136-42. Storry JR. Review: the LW blood group system. Immunohematology 1992;8:87-93) reports new information on the distribution of genetic variants in ICAM4 and reviews the complex serologic identification of the high-prevalence LWEM antigen. The role of ICAM4 in sickle cell disease and malaria susceptibility is discussed.