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Chemical structure of N-acetyllactosamine, the base unit in I and i antigens

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. [1] 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. [2]

I and i antigens

Adult red blood cells express I antigen abundantly. [3] Developing fetuses and newborns express i antigen until around 13-20 months after birth, when I antigen starts to be expressed instead. [3] 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. [1]

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. [1] [3] LacNAc repeats are made by the enzymes B3GNT1 and B4GALT1. [4] The i antigen is made of linear repeats, while the structure of the I antigen is branched. [3] 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. [5] [6] The gene encoding I-branching enzyme is located on chromosome 6. [6]

Clinical significance

The function of I and i antigens are unknown but may be related to hematopoiesis, the production of blood. [6] The rapid conversion from i to I antigens after birth suggests that I antigen plays an important role in adult red blood cells. [3] 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. [1] Enhanced expression of i antigen is associated with conditions involving stress hematopoiesis such as leukemia and sickle cell disease. [7]

Transient autoantibodies against I antigen are common, especially after infection by Mycoplasma pneumoniae, and are rarely significant except in cold agglutinin disease. [1] Transient antibodies against i antigen are common after infectious mononucleosis and are also not clinically significant. [1] Antibodies which recognize both I and i antigens are termed anti-j antibodies. [1]

Cold agglutinin disease

Main article: Cold agglutinin disease

The autoantibodies involved in cold agglutinin disease are usually against I antigen. [8] The antibodies are usually IgM ( kappa subtype), unlike transient autoantibodies which are generally IgG. [1] 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. [1] [8]

Adult i phenotype

Rarely, individuals have the i antigen on their red blood cells into adulthood, known as the adult i phenotype. [1] This is due to the presence of a mutation in the GCNT2 gene which encodes the I-branching enzyme. [1] [3] These individuals have alloantibodies against the I antigen, though these are typically cold agglutinins and are unlikely to cause transfusion reactions. [2] [9]

The adult i phenotype is associated with congenital cataracts, most markedly in Japanese and Taiwanese people and least markedly in Caucasian people. [1] [6] 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. [10]

The adult i phenotype is inherited in a recessive manner. [1]

History

The I antigen was first described in 1956 and the i antigen was discovered in 1960. [1] I and i were the first discovered antigens which change significantly during human development. [4] The letter I was chosen to reflect the "individuality" of a person studied who lacked the I antigen. [6]

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. [1] This is not seen in non-primates: cats, dogs, or guinea pigs. [1]

References

  1. ^ a b c d e f g h i j k l m n o p 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.
  2. ^ a b 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.
  3. ^ a b c d e f 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.
  4. ^ a b "OMIM Entry - # 110800 - BLOOD GROUP, I SYSTEM; Ii". www.omim.org. Retrieved 2021-01-31.
  5. ^ 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.
  6. ^ a b c d e 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.
  7. ^ 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)
  8. ^ a b 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.
  9. ^ 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.
  10. ^ "OMIM Entry - * 600429 - GLUCOSAMINYL (N-ACETYL) TRANSFERASE 2, I-BRANCHING ENZYME; GCNT2". www.omim.org. Retrieved 2021-01-31.

External links

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