Novel insights into the role of immune cells in skin and inducible skin-associated lymphoid tissue (iSALT)

The skin is equipped with serial barriers that provide rapid and efficient protection against external intruders. Beneath the epidermal physical barriers of the stratum corneum and the tight junctions, the integrated immune systems in both the epidermis and the dermis act in a coordinated manner to protect the host. This immunological barrier is composed of various cells, including skin-resident cells, such as keratinocytes, dendritic cells, tissue-resident macrophages, resident memory T cells, mast cells, and innate lymphoid cells. Additionally, infiltrating memory T cells, monocytes, neutrophils, basophils, and eosinophils are recruited in support of the host immunity. In addition to discussing the role of each of these cellular populations, we describe the concept of skin associated lymphoid tissue (SALT), which reminds us that the skin is an important component of the lymphatic system. We further describe the newly discovered phenomenon of multiple cell gathering under skin inflammation, which can be referred to as inducible SALT (iSALT). iSALT contributes to our understanding of SALT by highlighting the importance of direct cell-cell interaction in skin immunity.