In this paper we propose a new dynamical model for describing the interactions between immune and tumor cells. This model captures the effects of the tumor cells on the immune system and viceversa, through predator-prey competition terms. Additionally, it incorporates the immune system's mechanism to produce hunting immune cells, which makes the model suitable for immunotherapy strategies analysis and design. Consequently, we propose an approach based on Lyapunov functions in order to compute domains of attraction of equilibria of interest.