A rare example of weakened expression of the Kell (K1) antigen.

Random typing of donor units revealed a donor whose red blood cells gave very weak reactions with all anti-Kell (anti-K1) antisera. All other Kell-system antigen was present in four generations of the donor's family. The red-cell morphology is normal in all family members with the weakened K1 antigen. A consistent hypothesis is that the weak antigen is inherited through a variant allele at the Kell locus and represents a quantitative, not a qualitative, change.