The Concept of Pathogenic TH2 Cells: Collegium Internationale Allergologicum Update 2021
T helper (T<sub>H</sub>) cells have evolved into distinct subsets that mediate specific immune responses to protect the host against a myriad of infectious and noninfectious challenges. However, if dysregulated, T<sub>H</sub>-cell subsets can cause inflammatory disease. Emerging evidence now suggests that human allergic disease is caused by a distinct subpopulation of pathogenic T<sub>H</sub>2 cells. Pathogenic T<sub>H</sub>2 cells from different type-2-driven diseases share a core phenotype and show overlapping functional attributes. The unique differentiation requirements, activating signals, and metabolic characteristics of pathogenic T<sub>H</sub>2 cells are just being discovered. A better knowledge of this particular T<sub>H</sub>2 cell population will enable the specific targeting of disease-driving pathways in allergy. In this review, we introduce a rational for classifying T<sub>H</sub> cells into distinct subsets, discuss the current knowledge on pathogenic T<sub>H</sub>2 cells, and summarize their involvement in allergic diseases.