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 structure of the I antigen is branched." Unlike most other blood groups, the two antigens are not encoded by different alleles; rather, I-branching enzyme converts i antigen to I antigen by adding branches. The gene encoding I-branching enzyme is located on chromosome 6.
Clinical significance※
The function of I and i antigens are unknown. But may be related to hematopoiesis, the production of blood. The rapid conversion from i to I antigens after birth suggests that I antigen plays an important role in adult red blood cells. The presence of the linear i antigen in fetuses, rather than the branched I antigen, may have developed as an evolutionary mechanism to prevent ABO hemolytic disease of the fetus and newborn. Enhanced expression of i antigen is associated with conditions involving stress hematopoiesis such as leukemia and sickle cell disease.
Transient autoantibodies against I antigen are common, especially after infection by Mycoplasma pneumoniae, and are rarely significant except in cold agglutinin disease. Transient antibodies against i antigen are common after infectious mononucleosis and are also not clinically significant. Antibodies which recognize both I and i antigens are termed anti-j antibodies.
Cold agglutinin disease※
The autoantibodies involved in cold agglutinin disease are usually against I antigen. The antibodies are usually IgM (kappa subtype), unlike transient autoantibodies which are generally IgG. Cold-reactive IgM antibodies (cold agglutinins) bind to I antigen on red blood cells. And unlike IgG, are able to cause agglutination of red blood cells and activate complement to cause hemolysis, leading to anemia.
Adult i phenotype※
Rarely, individuals have the i antigen on their red blood cells into adulthood, known as the adult i phenotype. This is due to the presence of a mutation in the GCNT2 gene which encodes the I-branching enzyme. These individuals have alloantibodies against the I antigen, though these are typically cold agglutinins and are unlikely to cause transfusion reactions.
The adult i phenotype is associated with congenital cataracts, most markedly in Japanese and Taiwanese people and least markedly in Caucasian people. Cataracts occur when i antigen rather than I antigen is present on the epithelium of the lens, due to a mutation in the form of the I-branching enzyme which is expressed in lens epithelium, IGNTB.
The adult i phenotype is inherited in a recessive manner.
History※
The I antigen was first described in 1956 and the i antigen was discovered in 1960. I and i were the first discovered antigens which change significantly during human development. The letter I was chosen to reflect the "individuality" of a person studied who lacked the I antigen.
Other species※
A similar blood group system with a developmental change resembling the Ii system (with human neonatal cells expressing i antigen and adult cells expressing I antigen) has been observed in most primates, including chimpanzees and monkeys. This is not seen in non-primates: cats, dogs,/guinea pigs.
References※
- ^ Daniels G (2013-01-28). "I and i Antigens, and Cold Agglutination". Human Blood Groups. Oxford, UK: Wiley-Blackwell. pp. 469–484. doi:10.1002/9781118493595.ch25. ISBN 978-1-118-49359-5.
- ^ Castillo B, Dasgupta A, Klein K, Tint H, Wahed A (2018). "Red cell antigens and antibody". Transfusion Medicine for Pathologists. Elsevier. pp. 69–112. doi:10.1016/b978-0-12-814313-1.00005-8. ISBN 978-0-12-814313-1.
- ^ Yu LC, Lin M (November 2011). "Molecular genetics of the blood group I system and the regulation of I antigen expression during erythropoiesis and granulopoiesis" (PDF). Current Opinion in Hematology. 18 (6): 421–6. doi:10.1097/MOH.0b013e32834baae9. PMID 21912254. S2CID 205827249.
- ^ "OMIM Entry - # 110800 - BLOOD GROUP, I SYSTEM; Ii". www.omim.org. Retrieved 2021-01-31.
- ^ Pourazar A (January 2007). "Red cell antigens: Structure and function". Asian Journal of Transfusion Science. 1 (1): 24–32. doi:10.4103/0973-6247.28069. PMC 3168130. PMID 21938229.
- ^ Reid ME (2020). "The gene encoding the I blood group antigen: review of an I for an eye" (PDF). Immunohematology. 20 (4): 249–52. doi:10.21307/immunohematology-2019-458. PMID 15679458. S2CID 44662081.
- ^ Reid ME, Lomas-Francis C, Olsson ML (2012). "Ii Blood Group Collection". The Blood Group Antigen Facts Book. Elsevier. pp. 651–653. doi:10.1016/b978-0-12-415849-8.00037-5. ISBN 978-0-12-415849-8.
{{cite book}}:|work=ignored (help) - ^ Michalak SS, Olewicz-Gawlik A, Rupa-Matysek J, Wolny-Rokicka E, Nowakowska E, Gil L (November 2020). "Autoimmune hemolytic anemia: current knowledge and perspectives". Immunity & Ageing. 17 (1): 38. doi:10.1186/s12979-020-00208-7. PMC 7677104. PMID 33292368.
- ^ Poole J, Daniels G (January 2007). "Blood group antibodies and their significance in transfusion medicine". Transfusion Medicine Reviews. 21 (1): 58–71. doi:10.1016/j.tmrv.2006.08.003. PMID 17174221.
- ^ "OMIM Entry - * 600429 - GLUCOSAMINYL (N-ACETYL) TRANSFERASE 2, I-BRANCHING ENZYME; GCNT2". www.omim.org. Retrieved 2021-01-31.