Major depression is one of the most frequent psychiatric conditions. Despite many available treatment methods, more than 30% of patients do not achieve remission, even after trying several antidepressants and augmentation strategies. S-enantiomer of ketamine, well-known anesthetic and analgesic, has been recently approved by Food and Drug Administration in the intranasal form as a new generation antidepressant. However, the mechanism in which ketamine reduces depressive symptoms in treatment-resistant depression patients is still not completely understood. There are several theories explaining how ketamine might reduce depressive symptoms, which have been described in detail; one of them is immunomodulatory effect of ketamine, according to the inflammatory theory of depression. In the review authors present and summarize studies showing ketamine effect on human immune system ex vivo and in vitro, including changes in cytokine levels, number, ratio and activity of various immune cell population and the correlation with clinical improvement in depressive symptoms. Most of the results confirm the anti-inflammatory effect of ketamine. There are only a few studies in the population of patients suffering from depression receiving ketamine, focused on correlation between immunological changes and clinical outcome of the therapy; further studies of that area are neccesary for understanding the immunomodulatory effect of ketamine in depression.