Defining Th-cell subsets in a classical and tissue-specific manner: Examples from the skin

Th cells are important mediators of adaptive immunity and involved in various diseases. During the past decade, the Th family has expanded from including Th1 and Th2 cells to also encompass Th9, Th17, Th22, and Treg cells; the original classification using the expression of signature cytokines is still the gold standard for definition of subset affiliation. However, the identification of Th cells that do not fit into these tight conceptual boundaries has tumbled the field into an identity crisis. This review gives an overview on different Th-cell classification approaches, their advantages and drawbacks. In addition, this review highlights the functional properties of distinct Th subsets and their effector cytokines in tissues and disease-specific settings with a special focus on inflammatory skin diseases.