Here are two diseases that are easy to confuse: pemphigus vulgaris and bullous pemphigoid.

Both diseases are characterized by bullae (big, blister-like skin lesions) and both have “pemphig” in their names (“pemphig-” comes from the Greek pemphix, meaning blister, so that makes sense). So what are the differences between the two?

Pemphigus vulgaris

Here’s the key to differentiating between the two disorders. “Pemphigus” is used in a very specific way (you’d think it would be used to describe any blistering disorder, but not so!). It is used to describe blistering disorders caused by autoantibodies against the connections between cells of the epidermis.

Pemphigus vulgaris, not surprisingly, is the most common type of pemphigus (“vulgar-” comes from the Latin vulgaris, meaning the general public). It occurs primarily in adults between the ages of 30 and 60, and is characterized by big, flaccid bullae that burst easily (in most patients, you’ll see more ruptured, scab-covered bullae than intact ones). Patients often present first with oral bullae and ulcerations, and later develop bullae on the skin.

In this disease, patients have autoantibodies against desmogleins, which are part of the spot desmosomes (intercellular junctions that connect cells of the stratum spinosum with each other). The antibodies mess up the desmosomes, disrupting connections between the squamous cells of the epidermis and causing very superficial, intraepidermal, fragile bullae.

If you do immunofluorescence on the skin, you’ll see a kind of outlining of each individual epidermal cell (because there are autoantibodies bound to the junctions between the cells). Treatment consists of immunosuppressive agents; prognosis is variable, but many patients have a higher than normal mortality rate.

Bullous pemphigoid

This disease is called pemphigoid rather than pemphigus, because it looks like pemphigus but really isn’t! In pemphigus, there are autoantibodies against the connections between epidermal cells. But in bullous pemphigoid, the are directed against hemidesmosomes (specialized intercellular junctions that attach epithelial cells to the basement membrane).

This means that the bullae are actually subepidermal, so they are less fragile than those of pemphigus vulgaris (if you see a patient with bullous pemphigoid, you’ll see lots of intact, tense bullae, rather than a bunch of ruptured bullae covered with scabs). The immunofluorescence pattern is correspondingly different – you’ll see just a line at the base of the epidermis (rather than the lace-like outlining of epidermal cells you see in pemphigus vulgaris).

Patients with bullous pemphigoid are generally elderly, and the clinical presentation varies a lot (but usually it doesn’t start in the mouth, like pemphigus vulgaris). It’s a less serious disease, usually, since the bullae often don’t rupture (so there’s less chance of infection and scarring).

Bottom line: if you can remember that pemphigus is a disease that has intraepidermal antibodies, then you can keep the clinical presentation and immunofluorescence pattern of the two diseases straight.