Viruses are the most abundant and versatile pathogens which challenge the immune system and cause major threats to human health. Viruses employ different mechanisms to evade host immune responses that we describe them under the following headings: Inhibition of humoral responses, Interference with interferons, Inhibition and modulation of cytokines and chemokines, Inhibitors of apoptosis, Evading CTLs and NKs, and modulating MHC function. Viruses inhibit humoral immunity in different ways which contains change of viral antigens, production of regulatory proteins of complement system and receptors of the Fc part of antibodies. Viruses block interferon production and function via interruption of cell signaling JAK/STAT pathway, Inhibition of eIF-2α phosphorylation and translational arrest and 2'5'OS/RNAse L system. Also, Poxviruses produce soluble versions of receptors for interferons. One of the most important ways of viral evasion is inhibition and manipulation of cytokines; for example, Herpsviruses and Poxviruses produce viral cytokines (virokines) and cytokine receptors (viroceptors). In addition, viruses change maturation and expression of MHC I and MHC II molecules to interrupt viral antigens presentation and hide them from immune system recognition. Also, they inhibit NK cell functions. In this review, we provide an overview of the viral evasion mechanisms of immune system. Since most viruses have developed strategies for evasion of immune system, if we know these mechanisms in detail we can fight them more successfully.

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