In contrast to masking, Sia molecules are directly involved in a variety of recognition processes. This may be the most important role of these monosaccharides. This was first noted in microorganisms, which mostly use cell surface carbohydrates for binding to their host cells (Ofek and Doyle, 1994; Kelm and Schauer, 1997). Sia are the most frequent ligands for pathogenic and non-pathogenic viruses, bacteria and protozoa. The best known and longest studied example are influenza viruses.Hirst (1942), observed the binding of influenza A virus to human erythrocytes and mucins of the respiratory tract. The attachment, mediated by a vira