The Lewis antigen system is a human blood group system based upon genes on chromosome 19 q13.3, (FUT2) and 19p13.3 (FUT3), which both have fucosyltransferase activity. There are two main types of Lewis antigens, Lewis a and Lewis b. They are red cell antigens which are not produced by the erythrocyte itself. Instead, Lewis antigens are a component of exocrine epithelial secretions, and are subsequently adsorbed onto the surface of the erythrocyte. However, absorption can only take place if the Se (secreter) gene is also present.[1]
It is possible to have any combination of Lewis a, Lewis b, both or neither antigens. The most common are Lewis a negative and Lewis b positive Le(a-b+). Having both antigens present on red cells is extremely rare and is thought to be caused by a weak expression of the secretor gene.[1]
The link between the Lewis blood group and secretion of the ABO blood group antigens was possibly the first example of multiple effects of a human gene.[2] The presence of fucosyltransferase converts the Lewis a antigen to Lewis b. This is the same fucosyltransferase which can convert membrane bound A, B or H antigens into soluble A, B and H, allowing the person to secrete them into body fluids. Therefore, people with Lewis a antigens will not secrete the A, B or H antigens (and are called ABH non-secretors), and the presence of the Lewis b antigen is only found in secreters. Lewis negative (Le a-, Le b-) can be either secretors or non-secretors.
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