Ii antigen system

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

The Ii antigen system is a

fetuses and newborns.[2]

I and i antigens

Adult red blood cells express I antigen abundantly.

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

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.

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.

transfusion reactions.[2][9]

The adult i phenotype is associated with

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

guinea pigs.[1]

References

  1. ^
    ISBN 978-1-118-49359-5
    .
  2. ^
    ISBN 978-0-12-814313-1
    .
  3. ^
    S2CID 205827249
    .
  4. ^ a b "OMIM Entry - # 110800 - BLOOD GROUP, I SYSTEM; Ii". www.omim.org. Retrieved 2021-01-31.
  5. PMID 21938229
    .
  6. ^
    S2CID 44662081
    .
  7. ISBN 978-0-12-415849-8. {{cite book}}: |work= ignored (help
    )
  8. ^
    PMID 33292368
    .
  9. 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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