dominant trait. Among Sd(a) positive individuals, the expression of the antigen ranges from extremely weak to extremely strong. Very strong expression of the antigen is referred to as a Sd(a++)phenotype. In addition to being expressed on red blood cells, Sd(a) is secreted in bodily fluids such as saliva and breast milk, and is found in the highest concentrations in urine. Urine testing is considered the most reliable method for determining a person's Sid blood type.[3]
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transfusion reactions associated with transfusion of Sd(a++) blood to people with anti-Sd(a) antibodies.[2]: 224 Sid was officially designated a blood group in 2019 after its genetic basis was discovered.[1]
Molecular biology
The Sd(a) negative blood type is caused by
Tamm-Horsfall glycoprotein.[5] It is also expressed in the stomach, colon, kidneys, and lymph nodes.[6]: 686 Newborns do not express Sd(a) on their red cells until about 10 weeks after birth (although they do express the antigen in their bodily fluids),[2]: 224 [3]: 507 and expression of Sd(a) antigen on red blood cells often decreases during pregnancy.[2]
: 224
Clinical significance
The expression of Sd(a) antigen in positive individuals is highly variable, and ranges from expression so weak that it is barely detectable, to expression so strong that the cells are agglutinated by plasma from most human donors (polyagglutination). Extremely strong expression of Sd(a) is denoted as Sd(a++).[3]: 505
The Sd(a++) phenotype is sometimes referred to as the Cad phenotype, after a 1968 paper that identified a novel antigen in members of the Cad family from Mauritius.
antigenic determinant and are likely synthesized by the same enzyme, but there may be structural and quantitative differences in the expression of the two substances.[5] The Cad/Sd(a++) phenotype, like the Sd(a) positive phenotype, shows autosomal dominant inheritance.[3]
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Anti-Sd(a) is a naturally occurring antibody, meaning Sd(a) negative individuals produce it without having been exposed to Sd(a) positive blood through
Approximately 91% of people test positive for Sd(a) through
blood typing, and 96% test positive through urine testing.[2]: 224 The Sd(a++) phenotype is rare, especially in Europe, but may be more common in East Asian populations.[3]
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Laboratory testing
Urine is considered the optimal specimen for Sd(a) phenotyping.
hemagglutination inhibition testing: anti-Sd(a) is added to the urine, followed by Sd(a) positive blood cells. If Sd(a) is present in the urine, it will bind the antibody and prevent the red blood cells from agglutinating.[6]
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Anti-Sd(a) is usually composed of
ficin, papain, and trypsin, and the antibody is resistant to treatment with dithiothreitol.[6]: 686 Guinea pig urine contains very high concentrations of Sd(a) antigen and is sometimes used to identify anti-Sd(a) antibodies by inhibition testing.[3]
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History
The antigen was named for Sid, an employee at the
Lister Institute whose red blood cells were used for antibody screening. They were found to react strongly with samples containing an unidentified antibody, which was later characterized as anti-Sd(a).[2]: 224 Although the Sd(a) antigen was named in 1967,[9][10] and had been studied since at least 10 years earlier,[3]: 505 it was only connected to B4GALNT2 in 2003[6]: 687 [11][12] and assigned to a blood group in 2019 following the discovery of the molecular basis of the Sd(a) negative phenotype. It had previously been classified as part of the ISBT 901 series of high-incidence antigens.[1]
