When one sees pustules histologically though, you are going to see them at different levels in the skin, in different conditions. Pustules basically represent a neutrophilic (rarely eosinophilic) collection and they may be subcorneal, they may be intraepidermal, they may be dermal, but most pustules in fact are going to be in the epidermis or subcorneal area. You can get pustules that are made up of eosinophils rather than neutrophils and there is a group of disorders that are associated with these eosinophilic pustules.
The quintessential viral pustular disorder is herpes simplex and it usually gives rise to a pattern of intraepidermal vesicles. There is necrosis of the keratinocytes because of the cytopathic effects of the virus with balloon degeneration of the keratinocytes. The other thing that you may notice histologically is acantholysis or separation of the cells. This again is due to the viral damage, but the important thing is to look for this balloon degeneration of the keratinocytes. This is the reason the intraepidermal lesions form in the first place. There is swelling of the cell and rupture. Sometimes herpes virus can be seen along the sides of hair follicles as well but again the same balloon degeneration will be seen. Often you can get some multi-nuclear giant cells and variation in the shape of the nucleus of degenerate cells because of oedema. Herpes zoster and Varicella virus give exactly the same histological features as the herpes simplex virus.
The other viruses that can occasionally give a pustular eruption are Orf and Milker's nodules. They give almost a pseudo blister, although there can be small pustules on the surface, but histologically what you see depends on the stage of evolution of the lesion. These lesions usually evolve over a six to eight week period. They are due to a paravaccinia poxvirus that is picked up from young sheep or goats, generally when they are being fed. Histologically you will see again vaccuolation in the epidermis, but what you will see this time are pink inclusion bodies. Often there is full thickness necrosis of the epidermis and it is this that gives the impression of a blister. There is also quite marked vascular dilatation under these lesions, often with extravasated red blood cells as well and this is what gives the bright red colour to these lesions. There is often lymphocytic and neutrophilic infiltrate as well and it is the neutrophils that can play a part in pustule formation. A characteristic feature you should look for though is the dermal papillary oedema and this adds to the illusion that a blister is being formed here. Exactly the same pathology occurs with Milker's nodules.
Pustules can also be a feature of bacterial infections particularly those associated with follicles, which is usually a staph folliculitis and those associated with staph impetigo, but often in these circumstances it is not so much pustules as oozing which is seen and crust formation. The histopathology in impetigo is accumulation of neutrophils in the subcorneal space and this is why the surface of these blisters or pustules is quite fragile. There is not really a thick covering of epidermis and so they tend to burst and form crusts very quickly. You should be able to see Gram positive cocci here and there will be an inflammatory infiltrate in the papillary dermis. If the staphylococcus is a toxin producing staph then you may get some acantholysis in the subcorneal space.
Candidiasis is often a secondary infection seen in flexural disorders such as psoriasis and intertrigo and some satellite lesions are seen in the surrounding skin. These pustules are made up of neutrophils in the stratum corneum or in a subcorneal distribution, but you should be able to see pseudohyphae budding yeasts in the stratum corneum as well indicating the yeast basis or the fungal basis for this infection, but again the inflammation is centred high either in or just under the stratum corneum itself. In the rare variant chronic mucocutaneous candidiasis, where there is an immune deficiency causing failure to mount an immune reaction to candida, then the histology is similar to superficial candidiasis but there is more epidermal acanthosis and spores and hyphae were easily found. Occasionally deep fungi will rupture onto the surface of the skin in an extravasation phenomenon and give rise to perilesional pustules, but the histopathology here will show the dermal granulomas and dermal abscesses associated with this type of condition and special stains may pick up the relevant fungus as in sporotrichosis and in chromoblastomycosis.
The other clinical variants of pustular psoriasis are the generalised Von Zumbush variant with acute systemic symptoms including fever and explosive onset.
Impetigo herpetiformis, is a pustular variant of psoriasis occurring in pregnancy.
Acropustulosis or acrodermatitis of Hallopeau, which is seen on the tips of the fingers and around the nails often causing quite marked dystrophy of the nails. Reiters syndrome can also give rise to very similar histopathology to localised pustular psoriasis of the palms and soles with psoriasiform epidermal hyperplasia and a thick overlying stratum corneum layer. The two can be histologically indistinguishable from localised pustular psoriasis. The glucagonoma syndrome gives rise to necrolytic migratory erythema and again this looks somewhat like Sneddon Wilkinson Syndrome with a spreading lesion coming out of the flexures with pustules at the edge, but the pustules are really quite fragile. This is due a glucagon secreting tumour of the pancreas. The rash is typically seen in the perineum, buttocks and groin. You get a psoriasiform hyperplasia but a classic feature is pallor of the cells of the upper epidermis and this same picture is also seen in acrodermatitis enteropathica and they may have common neutritional deficiency based etiologies. There is usually parakeratosis overlying this pallor and sometimes some subcorneal splits. If neutrophils invade the subcorneal splits it gives rise to the pustules at the advancing edge of these lesions.
The last pustular disorder to consider in infants is infantile acropustulosis. Now, this disorder is often seen in infants who have had scabies, because pustules of the palms and soles are quite common as a reaction to infantile scabies. Again it is an intraepidermal pustule with neutrophils, occasionally with some eosinophils and it can subsequently go up to be a subcorneal pustule. There are no bacteria or fungi seen and often it is difficult to visualise any scabies mite either. This problem can persist after infantile scabies has been cured.