Considering all these premises, as well as the significance of the regulation of cathepsins’ activity on allergic and inflammatory processes and the diagnostic potential of cystatin D, already suggested for mastocytosis in our previous study [13], this exploratory study aimed to investigate the cystatin D-C26 interactome in the saliva of patients with different forms of SM and to evaluate qualitative differences and similarities between SM patients and healthy controls (Ctrls). Here, CST5 is linked to systemic mastocytosis.