Nikolsky’s sign (skin sloughing with light pressure) is positive whenever the epidermal layers separate from one another. It is positive in Pemphigus Vulgaris, SJS and SSSS. It’s negative in Bullous Pemphigoid, because the “bubble” is completely underneath the intact epidermis.

Pemphigus Vulgaris - antibody-mediated (IgG) destruction of desmosomes (the desmoglein protein specifically). This is a type 2 hypersensitivity. The epidermal layers separate (acantholysis), but the basal cells are still secured to the BM. A bubble appears within the spinosum layers. The spinosum cells appear round and free floating, while the basal cells have a tombstone appearance. These flaccid blisters pop easily. You won’t see a lot of bullae, you’ll just see open wounds. It hurts! Immunofluorescence highlights around keratinocytes in a reticular fish-net pattern. Involves skin and mucosa (it usually starts in the mouth). (+) Nikolsky’s sign, where the skin sloughs off when you apply pressure. Treat with immunosuppressants. The open wounds get infected, so mortality is pretty high. 

Bullous impetigo is a blistering infection seen in kids. It's caused by a toxin released during some staph aureus skin infections. Histologically similar to PV, but clinically it's localized and without mucosal involvement.

Bullous Pemphigoid - antibody-mediated (IgG) destruction of hemidesmosomes. Also a type 2 hypersensitivity. The basal cells peel off of the BM, forming a sturdy blister below (“bullo”) the epidermis. Doesn’t involve mouth. Nikolsky’s is (-) since this is a sub-epidermal bubble. Immunofluorescence will show a clean line along the BM. Mortality is less than PV (less open wounds). Treat w/ immunosuppressants.