This experiment aimed to investigate the effect of dietary Nigella sativa on the cell-mediated immune response. Eighteen male Wistar rats were divided equally into a control group and treated groups that received black seeds at rates of 30 and 50 g/kg in the diet (Sa30 and Sa50 groups, respectively, for 30 days. The weight gain, feed intake, feed conversion ratio (FCR), and cell-mediated immune response were monitored after the injection of 0.1 mL of 10% phytohemagglutinin (PHA). The intumesce index, serum total antioxidant capacity (TAC), catalase (CAT), interleukin-12 (IL-12), gamma interferon (γ-IF) and tumor necrosis factor alpha (TNF-α) were determined. Histopathological examination and an immunohistochemistry analysis of splenic caspase-3 and CD8 were performed. Nigella sativa significantly improved the weight gain and FCR. Intumesce index of Sa50 group was significantly increased. Nigella sativa significantly increased TAC, CAT, IL-12, γ-IF and TNF-α. A histological examination of PHA-stimulated foot pads showed increased leukocyte infiltration and edema in a dose-dependent pattern. Splenic caspase-3 and CD8 showed significant decreases and increases, respectively, in the Sa30 and Sa50 groups. The results indicate that Nigella sativa seeds exhibit immunostimulatory function through their antioxidant potential, induction of cytokine production, promotion of CD8 expression and reduction of splenic apoptosis.