Interferons (IFNs) are glycoprotein cytokines, first discovered in the 1950s by Isaacs and Lindeman. They have immunomodulatory, antiproliferative and anti-infective actions and are released by host cells in response to pathogens or tumour cells. They are especially important in protection against viral infection. Three classes of IFNs are recognized (Types I–III IFN) and these differ in their protein sequence, receptors, genetic loci and cell types responsible for their production.