The first stage involves the production of the cytokines interferon-α and β (IFN-α and -β), tumour necrosis factor-β (TNF-a) and interleukin12 (IL-12).These cytokines are produced by a range of immune cells (e.g. macrophages, B and T lymphocytes) and non-immune cells (e.g. endothelium and fibroblasts).Whilst these molecules are not responsible for the clearance of virally infected cells, they are important in containing the infection and engaging other elements of the immune system in defence. IFN-α and -β, for example, directly inhibit replication of viruses in all cells whilst also activating natural killer (NK) cells.