Background: It is well established that there is a significant correlationbetween overweight/obesity and immune system function. However, it isnot clear how the weight loss after bariatric surgery would affect thefunction of the human immune system.Objectives: We aimed to conduct this systematic review to assess the currentknowledge on alteration of immune system after bariatric surgery.Methods: We reviewed all potentially related articles published betweenJanuary 2006 and September 2020 to review the obesity surgery literatureand immune system activity in Medline/PubMed.Results: The results of a review of studies showed that obesity is associatedwith a low-grade systemic inflammatory condition in which innate andadaptive immune cells increase the secretion of proinflammatory cytokines.Weight loss due to bariatric surgery leads to significant changes in adaptiveimmune cells. The number of CD4 + and CD8 + T cells decreases. DecreasedTh1/Th2 ratio also occurs after weight loss due to obesity surgery. Also, theratio of inflammatory and anti-inflammatory cytokines changes followingchange in immune cells ratio.