Figure 2.

Innate lymphoid cells (ILCs) and natural killer (NK) cells are important tissue‐resident cells and mediators of homeostasis within the lung. NK cells are recruited during infection, although potentially resident populations have been described. NK cells are cytolytic cells that produce granzymes that mediate killing of infected macrophages. NK cells themselves are susceptible to direct infection by influenza but remain important producers of interferon‐γ (IFN‐γ), which contributes to inflammation. ILC1s are non‐cytotoxic counterparts of NK cells. That produce IFN‐γ and are important mediators of inflammation and viral clearance. Dendritic cell (DC) ‐derived interleukin‐12 (IL‐12) and IL‐18 can transition ILC2s into ‘ILC1‐like’ cells which may play a pathological role in chronic obstructive pulmonary disease. ILC2s produce type 2 cytokines IL‐5 and IL‐13, which are important in allergic responses and helminth clearance in the lung. They also facilitate tissue repair through their production of amphiregulin. ILC3s are the most abundant ILC subset in the human lung. IL‐22 producing ILC3s are important for bacterial clearance and tissue repair, while IL‐17 from ILC3s contributes to inflammation but may also have an important role in protection from extracellular bacteria and fungi.