A method of increasing immunity of a subject by administering an antidepressant to the subject is disclosed in the present invention. Preferably, the antidepressant is mirtazapine. And further, another object of the present invention is related to a method of administering the abovementioned antidepressant to the subject for treating cancer. The present invention tries to find the mechanism of the tumor growth inhibition by mirtazapine, and it may be due to the alteration of the tumor microenvironment, which involves the activation of the immune response and the recovery of serotonin level.
