Infiltration of the upper and deep corium

Superficial nad deep, dense, sharply demarcated perivascular lymphocytic infiltrates, epidermis normal; in superficial form the infiltrates are limited to the upper dermis only and epidermis may show spongiosis and microvesiculation.

Perivascular lymphocytic infiltrate of variable density, slight spongiosis and parakeratosis. Sometimes slight admixture of eosinophils. Clinical appearance is important to establish the diagnosis.

Dense lymphocytic perivascular infiltrate, superficial and deep. Dermal mucin is usually increased. Epidermis is normal (unlike in lupus erythematosus).

Edema in upper and middle dermis, often histologically less conspicuous than clinical appearance; perivascular mixed imfiltrate containing neutrophils and/or eosinophiles.

Lymphocytic infiltrate usually reaching to the middle dermis, containing eosinphils and neutrophils; dermal edema, variable spongiosis, parakeratosis and mild acanthosis.

Dense dermal perivascular and interstitial infiltrate with many eosinophils, which are partially degranulated. Alteration of collagen of variable extent, often surrounded by epithelioid cells: flame figures. The infiltrate may reach deep into the dermis, subcutis and even deeper.

Superficial and deep infiltrate with eosinophils (+lymphocytes, plasma cells, mast cells), sometimes with edema.

Dense infiltration reaching deep into the dermis, usually separated from epidermis by the narrow layer of infiltrated corium (grenz zone). This infiltrate is composed of lymphocytes, histiocytes, neutrophils and eosinophils. Blood vessels are affected (leukocytoclastic vasculitis), therefore extravasation of erythrocytes and siderophages are common.

Epidermis is slightly acanthotic, sometimes infiltrated with neutrophils. Chronic vasculitis with many neutrophils, leukocytoclasia and dense perivascular infiltrate. Sometimes edema of the surrounding dermis.

In long lasting lesions can form extracellular deposits of cholesterol.

Edema of papillary dermis, widespread infiltrates composed of neutrophils; malignant cells sometimes present as well (and lymphocytes, eosinophils sometimes).

Histology is quite variable.

Aphtous lesions show epithelial defect and (sometimes deep) perivascular and diffuse inflammatory infiltrate with neutrophils.

Skin pustules show deep neutrophilic infiltrate.

Erythema nodosum-like lesions show early lymphocytic infiltration within the fat and deep dermis; later many neutrophils are present.

Vasculitis and other changes are being described as well

Deep and dense infiltrate with many neutrophils, abscess, blood vessels with fibrin deposits, ulcerations.