## 1 Introduction  
Diabetes mellitus is a risk factor for the development of several macrovascular or microvascular complications such as coronary heart disease, ischemic strokes, retinopathy, nephropathy, and neuropathy [1; 2]. The underlying mechanisms behind vascular complications are multifactorial and complex. Red blood cells (RBCs) are implicated in the pathomechanism of vascular disturbances [3]. They are highly sensitive to most pathological conditions, such as hyperglycemia, because they lose their organelles when mature and only preserve a few metabolic pathways [4]. Moreover, in a hyperglyam environment, glucose concentration increases in the RBC cytosol because of passive glucose influx via the insulin-independent glucose transporter, which leads to non-enzymatic glycosylation of structural and functional proteins and the formation of advanced glycation end products [5; 6]. The glycosylation of RBC membrane components causes irreversible cross-linking of cytoskeletal proteins, thereby reducing membrane fluidity, which leads to decreased relative lateral fluidity of proteins and lipids in the membrane structure. The changed fluidity of diabetic RBCs results in the decreased deformability of cells and increases their aggregation. The stiffened and rigid membrane and clumping of RBCs make it difficult for them to pass through the microvessels, causing microvascular complications [7; 8]. A well-known glycated protein is the glycated hemoglobin A1c (HbA1c). The analysis of HbA1c in clinical tests of diabetic patients is routinely used to determine long-lasting blood glucose exposure [9]. In addition, HbA1c was found to be an independent cardiovascular risk factor. The glycated Hb is more susceptible to degradation, which leads to the enhanced fragility of RBCs, which in turn release higher quantities of free heme and iron, leading to oxidative stress. Moreover, glycated Hb may contribute to endothelial inflammation and vascular dysfunction [10]. Earlier studies demonstrated that diabetic patients may have other alterations in various hematological parameters, including RBC indices such as RBC count, mean corpuscular volume (MCV), or red blood cell volume distribution width (RDW) [5]. In addition, the higher proportion of morphologically altered RBCs in diabetes is a known consequence of this pathological condition; however, only a few studies have focused on quantification of different types of abnormally shaped RBCs in diabetic and insulin-treated diabetic rats using blood smears [8; 11; 12]. A disturbance in the balance of antioxidants and oxidants leads to oxidative stress. Oxidative stress is an important deleterious agent in several pathological conditions, such as chronic hyperglycemia, inflammation, or aging. Although RBCs do not have mitochondria, which are the main source of reactive oxygen and nitrogen species (RONS) in most cells, RONS are produced continuously in RBCs because of the heme iron content and the high oxygen tension in arteries. The elevated production of advanced glycation end products in chronic hyperglycemia is a triggering factor due to the glyco-oxidative stress. The antioxidant defense system, which scavenges and detoxifies RONS, consists of enzymatic and non-enzymatic pathways, such as superoxide dismutase, catalase, glutathione peroxidase (GSHPx), glutathione reductase (GR)-dependent regeneration of glutathione and NADH-methhemoglobin reductase or reduced glutathione (GSH), uric acid, and vitamins A, C, and E. The main non-enzymatic antioxidant in RBCs is the tripeptide GSH, which exists either in reduced or oxidized (GSSG) form via the reversible oxidation of its active thiol. The reactions are facilitated by GSHPx and GR. The ratio of GSH/GSSG indicates the redox potential of the cell. Moreover, GSH has multiple protective roles in defense mechanisms. It can directly scavenge free radicals or play a role as a substrate during the detoxification of hydrogen peroxide and lipid hydroperoxides catalyzed by GSHPx and glutathione-s-transferase [13; 14].