Cytokines are considered important factors in the modulation of various immune responses. Among them, interleukin （IL）-21 is one of the major immune modulators, adjusting various immune responses by affecting various immune cells. It has been suggested that IL-21 may enhance autoimmunity through different mechanisms, such as development and activation of helper T （TH）-17 and follicular helper T （TFH） cells, activation of natural killer （NK） cells, enhancing B-cell differentiation and antibody secretion and suppression of regulatory T （Treg） cells. Actually IL-21 is a pleiotropic cytokine whose functions in protection and immunopathology during parasitic diseases have been explored in limited ways, and it has been revealed to elicit antitumor-immune responses in several tumor models.