Describe Nikolsky’s sign and its relationship to pemphigus vulgaris.

Pemphigus vulgaris involves only the upper layers of the epidermis; therefore, the blisters are very fragile, and typically patients present with only superficial ulcerations. Because of the fragility of the skin, one can apply lateral pressure with a finger to the intact skin around a lesion, causing the upper layer of the skin to become detached. This is called the Nikolsky’s sign, and it occurs in pemphigus and other superficial blistering diseases. This clinical sign can be helpful in differentiating superficial from deeper blistering diseases (e.g., bullous pemphigoid, bullous lupus erythematosus), in which Nikolsky’s sign is usually absent.