Any of a number of proteins (see cytokine) that increase the resistance of cells to attack by viruses. In mammals, 10 species of interferons have been discovered, falling into three type classes: type I includes α-interferons from white blood cells and β-interferons from connective tissue fibroblasts; type II consists solely of γ-interferon from T cells and natural killer cells (NK cells); and type III comprises the λ-interferons (also known as interleukins 28A, 28B, and 29). α- and β-interferons induce intrinsic resistance to viral infection in all cells by triggering the expression of genes that encode antiviral proteins. Moreover, they activate NK cells, which selectively kill virus-infected cells, and promote synthesis of MHC class I proteins by all cell types, thereby protecting uninfected cells from attack by the NK cells. γ-interferon is distinct from the other two classes in that it is not produced directly as a result of virus infection. Its actions include macrophage activation, increasing the expression of MHC molecules, and suppression of TH2 helper T cells. It is produced by TH1 cells and cytotoxic T cells. Interferons are produced commercially for therapeutic purposes using genetically engineered bacteria or human tissue culture. However, interferons also play a role in the routine maintenance of cells; this complicates treatment, which can lead to unwanted side effects.