Experimental lethal infection is accompanied by extremely high levels of serum TNF-α, high frequency of splenic TNF-α-producing CD8+ T cells, decreased Ehrlichia-specific CD4+ T-cell proliferation, low IL-12 levels in spleen, and decrease in the quantity of ehrlichial antigen-specific IFN-γ-producing CD4+ TH1 cells. This evidence concerns the gene IFNG and infection.