Figure 2.

Innate, innate-like and adaptive lymphocytes contribute to the tissue-resident population in different organs. CD4⁺ and CD8⁺ T cells can be activated by ligation of their T cell receptor by peptide-loaded MHC proteins on the surface of antigen-presenting cells in conjunction with costimulatory molecules and cytokine signals. The γδ T cells, NKT cells, and MAIT cells have a T cell receptor with a restricted specificity. As innate-like T cells, they display a “preactivated” phenotype and can be activated via T cell receptor ligation or directly by cytokine signals. Conventional NK cells and “helper-like” ILCs are grouped together as ILCs and can be activated by direct recognition of damaged cells via natural killer cell receptors (NCRs) and/or cytokine signals (cytokine receptors [CRs]). TCR, T cell receptor.