IL18 and periodontitis: The majority of previous studies showed increased levels of IL-18 in the crevicular fluid and saliva of patients with various forms of periodontal disease [34,35,36,37,38,39], whereas the expression of IL-18 in gingival tissue biopsy specimens did not differ significantly between patients with periodontal disease and healthy subjects, or was even decreased in some forms of periodontitis [40,41].