On the other hand, mucous epithelia that are in contact with the environment, such as in the tracheobronchial or gastrointestinal tract, produce a great variety of N- and O glycans on mucins, the defense barrier against a multitude of microorganisms, by
providing ligands for their binding, followed by elimination with the viscous mucus (Lamblin et al.,2001).In a publication on human siglec-9 expressed on granulocytes and monocytes,Angata and Varki (2000) describe that the gene of this lectin is located on one chromosome, along with five other siglecs. The presence of this cluster suggests that these genes emerged by repeated gene duplication at some time during vertebrate evolution. These duplication events may have been selected for new specific functions of different cell types. Although some Ig superfamily genes are highly conserved from nematodes to humans, siglecs are only found in animals of the deuterostome lineage, suggesting that their emergence during evolution was dependent on the constitutive expression of Sia in deuterostomes. However, more research on this topic is necessary, in lower echinoderms and also in the few animals of the protostome lineage in which Sia was found, as in Drosophilaand other insects, to elucidate the presence of, and to completely understand the role of, Siarecognizing lectins. But a link from siglecs toDrosophila may exist, since CD22 shows some similarity to fruit fly proteins (Angata and Varki, 2000).