Other inflammatory skin diseases
How a pathologist reads a rash by where the inflammation sits: T cells killing basal cells in lichen planus, erythema multiforme and SJS/TEN, leaky vessels round mast cells in urticaria, and plugged, inflamed follicles in acne and rosacea.
By the end, you can
- Name the main tissue reaction patterns of inflammatory skin disease and the compartment each affects.
- Explain how cytotoxic T cells attacking basal keratinocytes produce lichen planus and erythema multiforme.
- Explain why widespread keratinocyte death in SJS and TEN behaves like a burn, and how it is graded.
- Distinguish mast-cell urticaria and angioedema from bradykinin-mediated angioedema by mechanism.
- Relate sebum, follicular plugging, Cutibacterium acnes and inflammation to the lesions of acne, and contrast rosacea.
- Recognise the classic appearances, grades and tests of these diseases, and explain how each main treatment acts on its mechanism.
What it is
Inflammatory dermatoses are non-infectious inflammatory diseases of the skin. Pathologists group them by tissue reaction pattern: where in the skin the inflammation sits and what it does there. Beyond the spongiotic pattern of eczema and the psoriasiform pattern of psoriasis, this lesson covers the interface pattern (lichen planus, erythema multiforme, Stevens–Johnson syndrome and toxic epidermal necrolysis), dermal mast-cell disease (urticaria and angioedema) and disease of the pilosebaceous unit (acne vulgaris and rosacea).
3 types to tell apart
- Interface dermatitis
- Lymphocytes gather at the dermal–epidermal junction and kill basal keratinocytes: lichen planus, erythema multiforme and, at its most extensive, SJS/TEN. Cutaneous lupus also shows this pattern and is taught with the blistering diseases.
- Urticarial (dermal) reaction
- Mast cells beside dermal venules release mediators that make the venules leak, with few inflammatory cells: urticaria and mast-cell angioedema. Bradykinin causes a second kind of angioedema without wheals.
- Pilosebaceous (follicular) disease
- The hair follicle and its sebaceous gland are plugged and inflamed in acne vulgaris; in rosacea the facial vessels and innate immunity drive inflammation round the follicles.
US spelling: angioedema → angiedema, centred → centerd.
Normal function
The skin’s compartments, and how a biopsy reads a rash
The skin is built in compartments, and each reacts to injury in its own way. The epidermis is a stack of keratinocytes: the of dividing cells rests on a thin basement membrane, and its daughters mature upwards through the . The interlocks with the epidermis in ridges; and run through the dermis, and mast cells lie alongside them.
The and its form the pilosebaceous unit: the gland empties oily sebum into the follicle canal, which opens on the surface. These units are largest and most numerous on the face, chest and upper back.
Faced with a biopsy of an inflammatory rash, a pathologist first asks where the inflammation sits. Fluid between spinous cells is the spongiotic pattern of eczema; regular thickening of the whole epidermis is the psoriasiform pattern of psoriasis. This lesson adds three more: lymphocytes at the junction (interface), leaky dermal vessels with few cells (urticarial), and inflammation centred on the follicle. Blisters, vessel walls and the fat have patterns of their own, taught separately.
| Feature | Where it sits | What the pathologist sees | Examples |
|---|---|---|---|
| Spongiotic | Between spinous cells | Fluid pushing keratinocytes apart; small vesicles | Atopic and contact eczema |
| Psoriasiform | The whole epidermis | Regular thickening, parakeratosis, no granular layer | Psoriasis |
| Interface | Dermal–epidermal junction | Lymphocytes killing basal keratinocytes | Lichen planus, erythema multiforme, SJS/TEN, cutaneous lupus |
| Vesiculobullous | Within or beneath the epidermis | A fluid-filled split | Pemphigus, pemphigoid |
| Urticarial | Dermal venules and mast cells | Oedema with few cells | Urticaria, angioedema |
| Vasculitis | Small-vessel walls | Inflammation damaging the vessel wall | Skin vasculitis |
| Follicular | The pilosebaceous unit | Plugged or inflamed follicles | Acne vulgaris, rosacea |
| Panniculitis | Subcutaneous fat | Inflamed fat lobules or septa | Erythema nodosum |
Simplified; many diseases mix patterns, and the pattern narrows the diagnosis rather than naming it. Clinical findings and immunofluorescence complete it.
Underlying change · Step 1 of 5
Interface dermatitis: T cells attack the basal layer (lichen planus)
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Consequences · Step 2 of 5
From target lesions to sheets of dead epidermis: EM, SJS and TEN
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Disrupted function · Step 3 of 5
Urticaria and angioedema: leaky vessels round dermal mast cells
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Disrupted function · Step 4 of 5
Acne vulgaris: a plugged, colonised and inflamed follicle
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Disrupted function · Step 5 of 5
Rosacea: flushing vessels and overactive innate immunity
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Symptoms and signs
Symptoms, signs and findings
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Exam detail
Exam detail
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Treatment
How treatment works
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Recap
Recap
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .
Self-check
Check your understanding
The full lesson is part of Student Pro
It takes you through all 5 steps of the mechanism, explains the symptoms, signs and findings each step causes, adds the exam detail and how treatment works where the lesson has them, then recaps it and lets you check yourself with 10 self-check questions. The full pathophysiology library: guided lessons with self-check questions comes with Student Pro, with 100 tutor answers a month and the full available catalogue.
Free to read: the introduction and .