CD4⁺ T cells can recognize antigens expressed in virus-infected cells and act cytolytically, to produce antiviral cytokines or to co-ordinate antiviral immune responses. Viruses escape detection by CD4⁺ T cells by at least two mechanisms. First, and perhaps foremost, viruses inhibit induction of the expression of MHC class II genes by blocking interferon-γ (IFN-γ) signal transduction and expression of the MHC class II transactivator (CIITA). Second, viruses inhibit the MHC class II antigen presentation pathway by affecting the stability or intracellular sorting of class II proteins. It appears that this viral inhibition is designed to prevent presentation of endogenous viral antigens in virus-infected host cells, rather than presentation of exogenous antigens in ‘professional' antigen-presenting cells.