Sialic Acids as Antigenic Determinants of Complex Carbohydrates

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Abstract
Carbohydrates have been known for long as potent antigens, described in numerous publications and extensively being discussed in this book. It became evident in the last years that sialic acids play an important role in this field, too, although their exact function is not yet understood in every case (Reutter et al., 1982; Schauer, 1982; Schauer, 1983; Schauer, 1985). Scheme 1 gives a survey of the influence of carbohydrates in general and of sialic acids in special on immunological reactions. On the one hand carbohydrates act as antigens, e.g. as differentiation and onco-developmental antigens (Feizi, 1985), and on the other hand they can mask antigenic sites on proteins, lipids and carbohydrate oligo- and polymers and thus can represent “anti-antigens”. It should be mentioned already here that the latter role is often due to, or at least strengthened by, the presence of sialic acids in glycan chains. Such a masking effect of carbohydrate chains may be illustrated by Figs. 1 and 2, showing, from different views, branched, N-glycosidically linked oligosaccharide chains, which cover the protein parts of the glycoprotein molecules like clouds and in this way may mask antigenic sites of the protein part.

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