[Hand for the dermatologist]

Through its anatomical, topographical, and functional distinctiveness, more than all other parts of the body, the hand is the interface between humans and their environment. All types of stimulus produce cutaneous signs in the hands, notably cold, light, pressure, contact, etc. Exposure to light makes it the region where photodermatosis is expressed, but also a particular site for cutaneous carcinogenesis. The countless chemical substances that the hand encounters can create specific or particular diseases in this location (irritative dermitis, contact eczema, hyperkeratosis, atrophy, etc.). The hand is also the site of infectious dermatosis transmission (erysipeloid, orf, mycobacteriosis, etc.), sometimes exotic (chromomycosis, histoplasmosis), and the site of plant penetration (protothecosis or more commonly thorns and splinters). The complexity of its vascularization and its many bones, joints, and tendons explain why it is a preferred area for signs of systemic diseases (diverse forms of lupus, dermatomyositis, inflammatory rheumatism, etc.). The nail unit alone shows innumerable signs of exogenic diseases, but also reflects certain internal diseases localized here with often characteristic signs. Here more than anywhere else, dermatology opens out to all of medicine.