Skin anatomy and cellular constituents. The protection of the body from the external environment is provided by the multilayered structure as well as by the complex cellular composition of the skin. The epidermis is the outermost layer composed of different strata made of keratinocytes (KC) from the most exposed surface to the least differentiated deeper area: stratum corneum, stratum granulosum, stratum spinosum, and stratum basale. Immune cells that ensure immunosurveillance such as Langerhans cells (LC) and specialized cells that produce melanin such as melanocytes are found in the epidermis. The dermis is the intermediate layer composed of several specialised immune cells such as plasmocytoid dendritic cells (pDC), dermal dendritic cells (dDC), macrophages (MØ), natural killer cells (NK), innate lymphoid cells (ILC), and T cells responsible of the immune response. In addition, blood and lymphatic vessels are present throughout the dermis. The hypodermis (not represented) is the innermost layer constituted mainly of adipose tissue.