IFN-γ promotes macrophage activation facilitating defence against bacterial pathogens,56 and it has been shown that IFN-γ as well as tumour necrosis factor-α are important co-determinants of antibody-mediated protection against pneumonic plague.57 Consequently, the increased presence of IFN-γ+ CD4+ cells in conjunction with F1-V specific neutralising antibody production could augment opsonization and clearance of Y. pestis. The gene discussed is CD4; the disease is plague.