*2.1. Gut Microbiota and Immunotherapy*

Immunotherapy with anti-PD-1/PD-L1 is more successful in tumors with inflamed T cells than in tumors with insufficient T cells because the PD-1/PD-L1 axis is known to play a crucial role in regulating immune system function. Recent research has indicated that gut microbiota may influence the PD-1/PD-L1 axis and the development of innate and adaptive immune systems [22,29]. According to a univariate study, gut microbiota diversity, *Faecalibacterium* abundance, and *Bacteroidetes* diversity were the best indicators of immunotherapy effectiveness. The effect of *Faecalibacterium* on the treatment response was proven by the FMT of responders and non-responders to anti-PD-1 in mice. One study also discovered that the biggest predictor of response to anti-PD-1 medication was the ratio of advantageous to non-beneficial operational taxonomic units (OTUs) [30]. Patients with a baseline majority of *Faecalibacterium* and other *Firmicutes* had longer Progression Free Survival (PFS) than those with a baseline predominance of Bacteroidetes, according to a study by Chaput et al. that analyzed the feces of 26 melanoma patients receiving ipilimumab [31]. Zheng et al. demonstrated a connection between certain gut flora and the effectiveness of immunotherapy in the treatment of liver cancer. The study discovered that *Akkermansia* and *Ruminococcus* were more prevalent in the gut microbiota of responders in hepatocellular carcinoma patients following PD-1 inhibitor therapy [32].
