Close-up of the skin of a patient with drug-induced toxic epidermal necrolysis, or Lyell syndrome. This is a severe condition in which the top layer of skin (epidermis) detaches itself from the lower layers (dermis). Often caused by a reaction to medication, the keratinocyte cells in the lower epidermis undergo necrosis, or death. These cells specialise in holding the skin together. It's a rare but life-threatening skin condition.