PDCD1 and psoriasis: In addition, another study done by Nagui et al. [16] showed that the PD-1 tissue levels, measured by ELISA, were reduced in psoriasis patients compared to controls, similar to our results; however, the difference was not significant, while significantly reduced levels of serum soluble PD-1 were detected in psoriasis patients compared to controls, though PD-L1 was not assessed in the study.