MedGen

The i and I antigens of the I blood group system (110800) are carbohydrate structures carried on glycolipids and glycoproteins and are characterized as straight or branched glycochains composed of repeating N-acetyllactosamine (LacNAc) units, respectively. Conversion of i antigen into an I-active structure requires the activity of the I-branching enzyme, beta-1,6-N-acetylglucosaminyltransferase (GCNT2; 600429), which adds the decisive GlcNAc-beta-1-6 branch onto the straight poly-LacNAc chains. Expression of the i and I antigens on red blood cells (RBCs) is reciprocal and developmentally regulated. Adult human RBCs predominantly express I antigen, whereas fetal and neonatal RBCs predominantly express i antigen. After birth, I antigen levels increase gradually as i antigen levels fall, with the normal Ii status of adult RBCs reached after about 13 to 20 months. Mutations that specifically affect 1 of the 3 variants produced by the GCNT2 gene cause the rare adult i phenotype (see 110800), in which adult RBCs are rich in i antigen and contain low levels of I antigen. Mutations that eliminate all 3 GCNT2 variants cause the adult i phenotype with congenital cataract (review by Yu and Lin, 2011). [from OMIM]

A blood group related both to the ABO and P systems that includes several different antigens found in most people on erythrocytes, in milk, and in saliva. The antibodies react only at low temperatures. [from MeSH]