The Blood Group Ii System: A Carbohydrate Antigen System Defined by Naturally Monoclonal or Oligoclonal Autoantibodies of Man

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Abstract
Cold agglutinin disease, an acquired haemolytic disorder of man [16], ranks among those diseases whose detailed investigation has helped to reveal some interesting phenomena in immunology and cell biology. The cold reactive autoantibodies of this disease were originally thought to be directed toward a red cell antigen common to all human beings [90]. It was the urgent need to find unreactive red cells for transfusion into a patient with severe haemolytic anaemia due to cold agglutinins that resulted in an extensive screening of thousands of donors and the discovery of the antigen system termed Ii [111]. The antigen recognized by the patient's antibodies was present on the red cells of all but five in 22,000 donors. The term ‘I’ was introduced for the commonly occurring antigen and ‘I’ initially to denote the lack of antigen I. Antibodies with preferential reactivity with i type cells were soon discovered. An important observation was made on the developmental regulation of these antigens on red cells [80] when foetal red cells were shown to be rich in i and in the course of the first 18 months of life the amount of I antigen increased to adult levels while the amount of i diminished. In only rare adults did this developmental change not occur. The majority of cold autoagglutinins fit the description of ‘anti-I’ or ‘anti-i’; however, other specificities do occur, e.g., anti-Pr [93], anti-Gd [94] and some cold agglutinins which are difficult to type.