The carbohydrate specificity of lectins is established most conveniently by the Landsteiner hapten-inhibition technique (Landsteiner, 1962; Goldstein & Hayes, 1978; Goldstein & Poretz, 1986). In this method the inhibitory effectiveness of various mono- and oligosaccharides of known composition is compared in a recognized and convenient reaction of lectins, such as haemagglutination. Initially, on the basis of their reactivity with monosaccharides that differed in configuration at C-3 or C-4 of the pyranose ring, Mäkelä (1957) suggested that lectins could be divided into four classes. Thus, for example, concanavalin A, which reacts with D-mannose and/or D-glucose, belongs to group III. Soyabean lectin, whose specificity is for N-acetyl-D-galactosamine and/or D-galactose, is classified as group II. According to Mäkelä's scheme the L-fucose-binding lectins are members of group I (Fig. 1.1). However, no group IV-specific lectins have been found in Nature so far. In more recent studies, although still within the four basic classes of Mäkelä, the definition of the specificity of lectins has been further refined and extended. Accordingly, from their reaction with simple sugars, lectins are first classified into broad groups of lectin classes as either mannose/ glucose-specific, or N-acetylglucosamine-specific, or N-acetylgalactosamine/galactose-specific or fucose-specific lectins. This is then followed by a more precise classification based on extensive investigations with a great number of oligosaccharides of known composition and structure. From such studies of the most complementary carbohydrate structure, the recognition and binding site of the lectins within the four main classes, is obtained.

Lectins are known to react chiefly with the non-reducing end of oligoand polysaccharides.