i

The Ii blood group system classifies human blood based on the presence of antigens I and i on red blood cells. The i antigen is found on the red blood cells of the developing fetus and newborn infants. In contrast, the I antigen is present on the red blood cells of all adults.1

Within about 18 months of birth, the i antigen gradually converts to the I antigen. This transformation is facilitated by a protein known as an I-branching enzyme. While rare variants of the i antigen exist, natural antibodies to I are found in adults who possess the i antigen, often due to a gene mutation. Auto-antibodies to I are a common cause of cold antibodies in acquired hemolytic anemia, and auto-antibodies to i have been identified in individuals with leukemia and other blood disorders.1