IL18 and infection: Previous studies looking at cytokines involved in Legionella immunity in immune-competent individuals found that Legionella-positive individuals had high serum levels of TNF-α, IFN-γ, and IL-12 during infection [13], while in vitro and in vivo studies reported elevated levels of TNF-α, IFN-γ, IL-2, IL-4, IL-6, IL-8, IL-12, IL-18, and MCP-1 [15,18,33].