**Abstract**

Glycans, as potential prognostic biomarkers, deserve attention in clinical glycomics for diseases diagnosis. The variety of glycan chains, attached to proteins and lipids, makes it possible to form unique glycoconjugates with a wide range of cellular functions. Under leukocyte-endothelial interaction, not only the availability of glycoconjugates with sialic acids at the terminal position of glycans are informative, but also the type of glycosidic bond by which sialic acids links to subterminal carbohydrates in structure of glycans. The process of sialylation of leukocyte glycoconjugates undergoes considerable changes in type 1 diabetes mellitus. At early stage of disease without diabetic complications, the pathology is accompanied by the increase of α2,6-linked sialic acids. The quantity of sialic acid-containing glycoconjugates on leukocytes surface increases in condition of disease duration up to five years. However, the quantity of sialic acids linked by α2,6-glycosidic bonds decreases in patients with the disease duration over ten years. Therefore, sialoglycans as marker molecules determine the leukocyte function in patients with type 1 diabetes mellitus, depending on the disease duration. Changes in the glycans structure of membrane glycoconjugates of leukocytes allow understanding the mechanism of diabetic complications development.

**Keywords:** sialic acid, glycans, glycoconjugates, sialyltransferases, sialidases, leukocytes, type 1 diabetes mellitus
