N- acetylneuraminic acid sialic acid (N- acyl neuraminic acid) is widely distributed in nature, which is mainly composed of glycoproteins, glycolipids or bacterial capsular substances in animal cell membranes or secretions. In glycoproteins or glycolipids, the keto group at position 2 forms glycosidic bond, which makes it located at the non-reducing end of sugar side chain. Its negative charge or unique chemical structure makes it have various physiological specificities.