The skin is the body’s largest organ and it’s also the first line of defence to the outside world. It’s made up of three main layers: the hypodermis (deepest layer), the dermis (middle layer) and the epidermis (upper layer). The outermost layer of the epidermis is called the stratum corneum and it forms the ‘skin barrier’, a complex structure whose job is to keep moisture in and allergens and irritants out1. The skin cells in the stratum corneum—known as corneocytes— are actually dead and they form only one part of the skin barrier.