Type 1 diabetes mellitus (T1DM) is a multifactorial, chronic, and autoimmune disease that results from a lack of endogenous insulin secretion from pancreatic β-cells. There are two markers currently available that indicate long-term diabetes control – HbA1c and fructosamine, but both have their limitations. Therefore, there is a great need for a marker that would show the longitudinal progression of T1DM in a more precise way. Changes in the N-glycosylation profile of total plasma proteins and immunoglobulin G in many studies have proven to be a potential prognostic, diagnostic and therapeutic marker of various diseases, especially those of an inflammatory nature, such as T1DM. Glycosylation of immunoglobulin A in various diseases has generally been less studied, and only a few years ago, intensive research into the role of its glycosylation in T1DM began. The aim of this study is to identify changes in IgA N-glycosylation in the population of adult patients with T1DM and compare them with healthy individuals. Glycan analysis was done by ultra performance liquid chromatography based on hydrophilic interactions. In this thesis, it was discovered that in adult subjects diagnosed with T1DM there is a statistically significant change in the proportion of glycan structures GP2, GP4, GP5, GP7, GP13, GP14, GP17, GP19, GP21, GP24, GP25, GP27 and GP29. A more detailed study of the derived traits showed a decrease in the proportion of low-branched glycans, and an increase in highly branched glycans. Furthermore, a reduction in the proportion of digalactosylated glycans and an increase in the proportion of trigalactosylated and trisialinised glycans is found in T1DM patients. The proportion of glycans with core fucose was significantly reduced in T1DM patients, while the proportion of high mannose glycans was significantly increased. These changes in IgA glycosylation indicate that it has a possible role in the disease progression and the need for further examination of its prognostic value.