Anguilla anguilla agglutinin (AAA), a fucolectin found in the serum of European eel, participates in the recognition of bacterial liposaccharides by the animal innate immunity system. Because AAA specifically recognizes fucosylated terminals of H and Lewis (a) blood groups, it has been used extensively as a reagent in blood typing and histochemistry. AAA contains a newly discovered carbohydrate recognition domain present in proteins of organisms ranging from bacteria to vertebrates. The crystal structure of the complex of AAA with alpha-L-fucose characterizes the novel fold of this entire lectin family, identifying the residues that provide the structural determinants of oligosaccharide specificity. Modification of these residues explains how the different isoforms in serum can provide a diverse pathogen-specific recognition.