The<unk>  is a human blood group system. It is based upon two genes on chromosome 19: FUT3, or<unk>  gene; and FUT2, or Secretor gene. Both genes are expressed in glandular epithelia. FUT2 has a dominant allele which codes for an enzym (designated Se) and a recessive allele which does not produce a functional enzyme (designated se). Similarly, FUT3 has a functional dominant allele (Le) and a non-functional recessive allele (le). The proteins produced by the FUT2 and FUT3 genes modify type I oligosaccharide chains to create<unk>  antigens. These oligosaccharide chains are similar to the type II chains of the ABO blood system, with a single bond in a different position. The link between the<unk>  blood group and secretion of the ABO blood group antigens was possibly the first example of multiple effects of a human gene: the same enzyme (fucosyltransferase2) which converts the Le-a antigen to Le-b is also responsible for the presence of soluble A, B and H antigens in bodily fluids. There are two main types of<unk>  antigens,<unk>  a (Le-a) and<unk>  b (Le-b). There are three common phenotypes: Le(a+b-), Le(a-b+), and Le(a-b-). The enzyme fucosyltransferase 3 (FUT3), encoded by Le gene, adds a fucose to the precursor oligosaccharide substrate, converting it to the Le-a antigen. People who have the Le allele and who are non-secretors (homozygous for the nonfunctional se allele) will express the Le-a antigen in their bodily fluids and on their erythrocytes. If a person has both the Le and Se alleles, their exocrine cells will also have the enzyme fucosyltransferase 2 (FUT2). This adds fucose to the oligosaccharide precursor in a different position from the FUT3 enzyme. This produces the Le-b antigen. In most people having both Le and Se, it is difficult to detect the antigen Le-a. This is because the activity of the FUT2 enzyme is more efficient than the FUT3 enzyme, so the type I oligosaccharide chain is mostly converted into Le-b instead of Le-a. Therefore, people with readily detectable<unk> -a antigen are non-secretors; they do not have FUT2 activity.<unk> -b antigen is found only in secretors: people who possess the Se allele and thus have FUT2 activity.<unk>  negative people (Le a-, Le b-) are homozygous for the recessive le allele and can be either secretors or non-secretors.
Lewis antigen system
MNS antigen system
Kell antigen system
Lutheran antigen system
Kidd antigen system
Colton antigen system
Yt antigen system
Ii antigen system