The invention provides a method of treating or preventing viral diseases in a mammal comprising administering to the mammal an interleukin (IL)-21 blocking agent in an amount effective to treat or prevent the viral disease in the mammal. Also provided is a method of reducing the activation or recruitment of immune cells in a mammal comprising administering to the mammal an IL-21 blocking agent in an amount effective to reduce the activation or recruitment of immune cells in the mammal. Methods of decreasing the expression of at least one cytokine or at least one protein in a mammal comprising administering to the mammal an IL-21 blocking agent in an amount effective to decrease the expression of the cytokine or the protein are also provided.
