Blocking interleukin (IL-21) may be an effective method to treat or prevent various viral infections. In the course of an immune response to a virus, IL-21, produced primarily by CD4+ T cells, can inhibit or stimulate (regulate), immune cell function (B cells, T cells, natural killer cells, dendritic cells). IL-21 regulation may be either protective or pathological; autoimmune disease pathology has been associated with IL-21 promoted inflammation (in: type 1 diabetes, lupus, and multiple sclerosis). This technology describes methods of blocking IL-21 that may reduce damaging inflammatory responses during certain viral infections. Specifically, the absence of IL-21 during respiratory viral infection such as pneumonia virus infection actually prevents some of the pathogenic effects that may be promoted by IL-21. Methods for controlling IL-21 signaling may be used to treat to prevent many pathological effects of pneumonia viruses, and other viral infections.