Pustules are yellowish, while vesicles or bullae may contain clear, serous fluid. These are considered the primary lesions and are very fragile, hence more often you will see the lesions present as crusts - what remains after the lesion ruptures. If the epidermal separation is at a deep level, you can see erosions and ulcerations. The entire epidermis is separated from the dermis. TEN Bullous pemphigoid Low Power High Power Positive Immunofluorescence.
Microscopic example given is skin from a horse with pemphigus foliaceous.
The surface of the pustule is just the fragile stratum corneum. You can see why the fragile pustules are usually transient and give way to the more commonly encountered secondary lesion of a crust. This is an example of a type II hypersensitivity disease. Auto-antibody deposition can be detected via special procedures such as immunoperoxidase staining or immunofluorescence. Gross lesion examples from a horse with pemphigus foliaceous.