Skip to content
Explore atlas
PH-SD25

Skin vasculitis and panniculitis

How immune complexes in the dermal venules cause palpable purpura and lasting wheals, why vessel depth reveals vessel size, and how inflammation of the fat septa (erythema nodosum) differs from lobular panniculitis that kills fat.

  • Free preview
  • 5 steps · 10 questions
  • 3D normal anatomy

By the end, you can

  • Explain how immune complexes in the superficial dermal venules cause leukocytoclastic vasculitis and palpable purpura, and why the lesions follow a drug or infection by a week or two.
  • Relate the depth of an inflamed skin vessel to its size and to purpura, livedo, nodules and ulcers.
  • Recognise IgA vasculitis and urticarial vasculitis in the skin and what each signals beyond it.
  • Distinguish septal panniculitis (erythema nodosum) from lobular panniculitis (erythema induratum, pancreatic and lupus panniculitis) by mechanism, microscopy and course.
  • Name the biopsies and tests that type skin vasculitis and panniculitis, and explain how the main treatments act on the mechanism.

What it is

Skin vasculitis is inflammation of the walls of skin vessels, most often the small venules of the upper dermis (cutaneous small-vessel or leukocytoclastic vasculitis), either alone or as part of a systemic vasculitis. Panniculitis is inflammation of the subcutaneous fat, described by whether it centres on the fibrous septa or on the fat lobules, and whether vessels are inflamed too.

5 types to tell apart
Cutaneous small-vessel (leukocytoclastic) vasculitis
Immune complexes in the superficial dermal venules: crops of palpable purpura on the legs, often after a drug or infection, usually limited to the skin.
IgA vasculitis in the skin
IgA-dominant deposits in the venule walls; purpura on the buttocks and legs that can signal gut, joint and kidney involvement.
Urticarial vasculitis
Wheals that last, burn and bruise. Normocomplementaemic forms are usually limited to the skin; the hypocomplementaemic form (anti-C1q) can be systemic and is linked with lupus.
Septal panniculitis
Inflammation of the fibrous septa of the fat without vasculitis: erythema nodosum, which heals without scarring.
Lobular panniculitis
Inflammation of the fat lobules with death of fat cells: erythema induratum (with vasculitis, linked with tuberculosis), pancreatic panniculitis and lupus panniculitis.

US spelling: centres → centers, leucocytes → leukocytes, centred → centerd.

Normal function

The skin’s vessels, and the fat beneath it

The skin has two horizontal networks of vessels: a deep plexus where the dermis meets the fat and a superficial plexus just beneath the dermal papillae, joined by vertical vessels. Each papilla holds a capillary loop that drains into the postcapillary venules of the superficial plexus, where leucocytes normally leave the blood and where circulating immune complexes most easily lodge. The larger muscular and run deeper, in the fat.

The subcutaneous fat (the panniculus) is built of of fat cells separated by fibrous . The septa carry the arteries, veins, lymphatics and nerves; each lobule is fed by a small arteriole from a septum that branches into capillaries round every fat cell, and drains back to septal veins. In the usual explanation, a lobule has little collateral supply, so injury to its septal vessels can starve the whole lobule.

This layout lets a pathologist read inflammation by where it sits. Inflammation centred on vessel walls is vasculitis, and the depth of the vessel shows its size: small vessels of the upper dermis, or medium arteries where the dermis meets the fat. Inflammation of the fat itself is panniculitis, mainly septal or mainly lobular, with or without vasculitis.

Point to it on the model