PDCD1LG2 and mastocytosis: Additionally, in 2016, Rabenhorst et al. examined PD-1, PD-L1, PD-L2, and tryptase within the serum of 43 patients with mastocytosis and 22 healthy controls, demonstrating significantly increased PD-L1 within the serum of adult patients with mastocytosis, compared to controls.