Besides the ABO and the Rh system there are a number of other unrelated blood group antigen systems that have some importance, either for medicolegal parenthood studies or because sensitization to these antigens causes transfusion reactions or hemolytic disease of the newborn. The most important of these systemsis Kell (K), a well-recognized blood bank problem. The Kell antibodies are similar in characteristics and behavior to the Rh system D (Rho) antibody. Fortunately, only about 10% of whites and 2% of African Americanshave the Kell antigen and are thus Kell positive, so that opportunities for sensitization of a Kell-negative person are not great. Kell antibody is univalent and, like Rh, acts best in vitro in a high-protein medium at 37°C. If reactions to Kell antibodies occur, results of the direct Coombs’ test are positive (until the affected RBCs are destroyed). A similar situation exists for the rare Duffy (Fy) and Kidd (jK) systems. There are other systems that resemble the ABO system in their antibody characteristics, and these include the MN, P, Lewis (Le), and Lutheran (Lu) systems. They are primarily bivalent antibodies and react best in vitro in a saline medium at room temperature or below. They are rare causes of transfusion reactions, and when difficulties arise, they are clinically milder than the problems associated with the univalent antibody systems.

There are a large number of so-called minor blood group antigens. The most important is the I-i, or IH, system. This system is very weak in newborn RBCs and becomes established at approximately age 2 years. Anti-I (or anti-IH) antibodies are IgM cold agglutinins. Anti-I antibody is a frequent cause of viral or mycoplasma-associated cold agglutinins or idiopathic cold autoimmune hemolytic anemia. Some of the other systems include “public,” or “universal,” antigens, such as the Vel system. Nearly all persons have the antigen on their RBCs, so the chance is extremely small that any individual would not have the antigen and thereby would be capable of producing antibody to the antigen. Another group includes the “private,” or “low-incidence,” antigens, the most common of which is the Wright (Wra) antigen-antibody system. Very few persons have the low-incidence antibody on their RBCs, so risk of encountering the antigen is extremely small. Yet another antigen group is the “high titer–low avidity” type of antibody, which is occasionally responsible for weakly positive Coombs’ test results inrecipient or donor antibody screens. In the vast majority of cases other than high-titer anti-I antibodies, the minor group antigens or antibodies are not clinically significant. However, they are a great source of frustration to the blood bank when they occur, because any unexpected antibody must be identified and because they interfere with the crossmatch and cause donor bloods to appear incompatible with the recipient.

In summary, RBC typing is ordinarily designed to show what ABO and Rh antigens are on the RBC and thus what blood group RBCs can be given to a recipient, either without being destroyed by antibodies the recipient is known to possess or without danger of sensitizing the recipient by introducing antigens that he or she might lack and against which he or she might produce antibodies.