H antigen

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The H antigen (substance H, O-secretor mucin glycan phenotype) is the common intermediate substrate for the A and B antigens of the ABO blood groups[1]. Its antigenicity is defined by the terminal disaccharide Fucose-Galactose - (α-1-Fuc-(1 → 2)-β-d-Gal-O-R, where R is glycolipid or glycoprotein). Its expression by epithelium is associated with susceptibility to asthma exacerbations[2]. It is formed by the enzyme Galactoside 2-alpha-L-fucosyltransferase 1 and non-functional mutation of its FUT1 gene results in the Bombay blood group which can not express A, B or H antigens and who must not receive blood tranfusions from anyone apart from other Bombay blood group members.