Ii Antigen System
The Ii antigen system is a human blood group system based upon a gene on chromosome 6 and consisting of the I antigen and the i antigen. The I antigen is normally present on the cell membrane of red blood cells in all adults, while the i antigen is present in fetuses and newborns. I and i antigens Adult red blood cells express I antigen abundantly. Developing fetuses and newborns express i antigen until around 13-20 months after birth, when I antigen starts to be expressed instead. Like ABH antigens, which make up the ABO blood group, I and i antigens are not restricted to the red blood cell membrane, but are found on most human cells and in body fluids such as saliva. The I and i antigens are carbohydrate structures composed of repeating units of ''N''-acetyllactosamine (LacNAc), and are located on the interior of structures carrying ABH and Lewis antigens. LacNAc repeats are made by the enzymes B3GNT1 and B4GALT1. The i antigen is made of linear repeats, while the structur ...
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