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Kpᵃ (KEL3), Kpᵇ (KEL4) and Kpᶜ (KEL21) are part of the Kell system (ISBT, number 006). Kpᵃ is present in approximately 2% of the Caucasian population while Kpᵇ is a high frequency (public) antigen. The Kpᶜ antigen has an occurrence of less than 0.01% but up to 0.32% in Japanese. Anti-Kpᵃ and anti-Kpᵇ are less common than anti-K but have similar serological characteristics and are considered to be clinically significant. They may occur after transfusion or by feto-maternal immunization.