Figure 1.

Key phenotypic properties and location of skin‐resident T cells. Phenotypic characteristics of skin‐resident memory T cells and distribution in human skin are shown. All memory T cells express CD45RO, but the absence of CCR7 and expression of CD69 and CD103 distinguish T_RM cells from circulating memory T cells. CD49a is found on epidermal CD69⁺ CD103⁺ CD8⁺ T cells only. CD69 and CD103 can be found on both CD4⁺ and CD8⁺ T cells, but at different levels. CD4⁺ T cells constitute approximately 75% of the lymphocytes present in both layers of the healthy human skin. However, there are twice as many CD103⁺ T_RM cells (both CD4⁺ and CD8⁺) in the epidermis, while in the dermis, the majority of T_RM cells are CD103⁻. −, no expression; + expression. Regarding CD4⁺ and CD8⁺: + represents low expression frequency; ++ medium expression frequency; +++ high expression frequency. * Fraction of CD69⁺CD4⁺ and CD69⁺CD103⁺CD4⁺ T_RM cells of the total CD4⁺ T_RM cell pool in either the epidermis or dermis are shown. **Fraction of CD69⁺CD8⁺ and CD69⁺CD103⁺CD8⁺ T_RM cells of the total CD8⁺ T_RM cell pool in either the epidermis or dermis are shown