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Citation Information : Immunohematology. Volume 20, Issue 4, Pages 249-252, DOI: https://doi.org/10.21307/immunohematology-2019-458
License : (Transfer of Copyright)
Published Online: 18-May-2020
Unlike most blood group antigen pairs, the I and i antigens are not antithetical (produced by allelic pairs) but, rather, they are reciprocal. The I antigen is formed by the action of an enzyme (a glycosyltransferase), which adds branches onto the i antigen. Thus, branched I antigen is formed at the expense of its precursor, the linear i antigen. The antigens are present on all blood cells and have a wide tissue distribution. Soluble I antigen is found in milk, saliva, and amniotic fluid, and a small amount is in plasma. The function of these antigens is unknown but the I antigen has a decreased expression and the i antigen has a concomitant increased expression in conditions that result in increased hematopoiesis. The gene encoding the branching transferase has been cloned and sequenced, and the mechanism underlying the i adult phenotype with and without association with cataracts has been elucidated.