Immunogenicity is the ability of an antigen to provoke an immune response in an antigen-negative recipient. Why some antigens are more immunogenic than others is unknown.
Other than antigens in the ABO system, Rh (D) is the most immunogenic red cell antigen, followed by K in the Kell blood group system. Other immunogenic antigens include c and E in the Rh system.
In routine blood banking, assessments of an antigen's immunogenicity are typically based on the prevalence of the corresponding antibody and do not take into account the frequency of the antigen in the general population. For example, k in the Kell system may be very immunogenic but anti-k is rare since 99.8% of Caucasians are k+ and cannot make anti-k.