## 1 Introduction
Most of these proteins play overlapping antibacterial, antiviral, and antifungal functions. A lack of sufficient amount of these proteins that fight oral infections may lead to higher susceptibility to this kind of oral infection. One of the most common opportunistic infections in patients with dry mouth is oral candidiasis. Xerostomic patients are at higher risk for oral Candida growth than controls and are at higher risk for developing oral candidiasis than controls [11; 12]. Patients with pSS presenting oral lesions have lower unstimulated and stimulated whole salivary flow rates than those without oral lesions [12]. Moreover, disturbed calcium-phosphate homeostasis and enamel demineralization resulting from decreased salivation cause increased caries rate [10]. The opposite salivary disturbance to xerostomia is sialorrhea defined as an increased amount of retained saliva in the oral cavity. It occurs either due to salivary overproduction or impaired clearance of saliva. Sialorrhea is commonly caused by swallowing impairment. It is a frequently reported non-motor symptom affecting 30-80% of patients with Parkinson's disease (PD) [13]. In these cases, there is a direct relationship between salivary gland malfunction and oral health. Moreover, chronic and persistent dry mouth significantly reduces patients' quality of life [14]. Unbearable dry mouth, especially at night, makes sleeping and resting difficult. The frequent occurrence of xerostomia in SS patients led to its inclusion in previous diagnostic criteria [15; 16; 17]. On the other hand, xerostomia was argued to be a subjective symptom and not included in the diagnostic criteria for SS. This realization has led to a gradual evolution of diagnostic criteria toward more objective symptoms and diagnostic tests. One such objective test for assessing salivation in current SS diagnostic criteria is sialometry in the form of the unstimulated whole salivary flow rate (USWSF) [17]. Another unresolved issue in SS is the source of hyposalivation. Traditionally, decreased saliva secretion in SS is attributed to morphological changes in the salivary glands resulting from the presence of lymphocytic infiltrations and severe fibrotic rebuilding. These morphological tissue changes are representative of all salivary glands, and their typical microscopic pattern in labial salivary glands, termed focal lymphocytic sialadenitis (FLS), is the basis for confirming histological SS and is one of the diagnostic criteria for SS. FLS damages glandular tissue and results decreases saliva secretion [18; 19]. Labial salivary gland biopsy sensitivity ranges from 79% to 91.4% and specificity from 81.8% to 100% [20]. However, there is no direct relationship between the severity and perception of xerostonia and the severity of morphological changes and inflammation of the salivary glands. Moreover, this does not consider other lesions such as acinar atrophy, epithelial cell proliferation, fibrosis, and small focal infiltrates [20]. Clinical observations have revealed that some SS patients present significant symptoms of xerostonia compared to a relatively low degree of inflammation in the salivary glands and tissue damage to a minor extent. Furthermore, some SS patients do not develop a more severe disease state or full-blown SS manifestation, only presenting symptoms of unbearable dry mouth [21]. In the previous study, some genes were recognized as responsible for differences in dry mouth severity in SS patients. Significant correlations were observed between Transmembrane P24 Trafficking Protein 10 (TMED10), Protein Disulfide Isomerase Family A Member 4 (PDIA4), and Calnexin (CANX) gene expression in SS patients suffering from sicca symptoms compared to controls. Among the genes changed in SS and responsible for saliva secretion, there is also a set of genes involved in ER function and the loss of cholinergic stimulation. It appears that xerostonia in SS has a multifactorial origin and may be independent of the tissue morphological changes in the salivary glands specific to SS [22].