ILC1s are a subset of innate lymphoid cells (ILCs) that has been classified along with natural killer (NK) cells as group 1 ILCs. In addition to this group, two other subsets of ILCs (ILC2s and ILC3s) have been described. All three subsets of ILCs are primarily localized to mucosal surfaces and are particularly important regulators of epithelial barriers. Although ILCs lack rearranged antigen receptors and therefore do not exhibit antigen specificity, each ILC subset that has been identified mirrors a specific T helper cell subset in terms of the molecules that they express and their functions. While NK cells are thought to be the innate counterpart of CD8+ T cells, ILC1s are considered to be the innate equivalent of Th1 cells. ILC1s are activated by IL-12, IL-15, and IL-18. Both ILC1s and NK cells are required for host defense against intracellular viral and bacterial pathogens, and express NK1.1, NKp46, T-bet, and IFN-gamma. Unlike NK cells, however, ILC1s express CD127/IL-7 R alpha and lack expression of the transcription factor eomesodermin (Eomes). Additionally, ILCs do not express cell surface molecules that are commonly used to identify other immune cell types, and therefore are defined as lineage negative (Lin-). In mice, ILC1s are defined as Lin-NKp46+NK1.1+CD127+T-bet+Eomes- cells that also typically express CD49a and CD122/IL-2 R beta. In humans, ILC1s have been described as Lin-CD127+CRTH2-CD117-NKp44-CD56-CXCR3+T-bet+Eomes-IFN-gamma+ cells. Additionally, ILC1s have not been found to express activating or inhibitory mouse Ly49 or human KIR family receptors that detect MHC class I molecules, and they do not produce cytotoxic molecules such as perforin or granzyme B.