Rather than a tired abbreviation for “just kidding” in a text message, Jk is a much more exciting shorthand for the Kidd Blood Group System! The name comes from the initials of “John Kidd,” a child of the woman in whom the first Kidd antibody was discovered. When Kidd antibodies were first identified, the Kell system (using abbreviation “K”) had already been described, so to avoid confusion, Kidd was assigned “Jk.”