Objective: To investigate the relationship between immune cell infiltration and postoperative recurrence of differentiated thyroid cancer.
Methods: The clinical data of 507 patients were downloaded from the TCGA database. The infiltrations of 22 different kinds of immune cells in the cancer tissues were estimated by deconvolution method, and the relationship between the degree of infiltration of each kind of immune cells and postoperative recurrence of the patients was analyzed by survival analysis. 
Results: The analysis of deconvolution showed that except the naive CD4 T was absent in all specimens, all the other 21 kinds of immune cells had varied degrees of infiltration in all specimens. The analysis of Kaplan-Meier survival curves and Log-rank tests showed that only the activated NK cell infiltration was significantly associated with the disease-free survival of the patients, as evidenced by the disease-free survival time in patients with high degree of activated NK cell infiltration was significantly longer than that in patients with low degree of activated NK cell infiltration (P=0.004). Multivariate Cox regression risk model showed that the degree of activated NK cell infiltration was an independent influential factor for postoperative recurrence of differentiated thyroid cancer (HR=0.17, 95% CI=0.05–0.62, P=0.007). 
Conclusion: Activated NK cells can reduce the risk of postoperative recurrence of differentiated thyroid cancer and prolong the disease-free survival of patients, which is an independent protective factor against postoperative recurrence of differentiated thyroid cancer.