Skin Immunologic - Pathology

()Map INTEGUMENTARY SYSTEM (Map)

General

• Classification:
  • Hypersensitivity - response to normally harmless substances
  • Auto-immune - antobodies or T-cells reactive against self-antigens
• Mostly involves mixture of types described below

Hypersensitivity reactions

Allergic Contact Dermatitis

• Usually involves Type IV reaction
• Pruritic lesions with self-inflicted trauma
• At areas in contact with allergen
• Grossly:
  • Erythema, papules, +/- vesicles, exudation -> crusts
  • If chronic, lichenification, hyperpigmentation, alopecia
• Microscopically:
  • Spongiotic superficial perivascular dermatitis
  • Mononuclear cells
  • If chronic, epidermal hyperplasia
  • May involve eosinophils

Atopy

• Involves Type I reaction
• Mainly causes pruritus
• Dogs - face rubbing and foot licking; secondary pyoderma or seborrhea
• Cats - facial, ear or generalised pruritus, miliary dermatitis, eosinophilic granuloma complex, symmetric alopecia
• Horses - pruritic hea, pinnae, ventrum, legs, tailhead or recurrent urticaria
• Lesions generally due to self-trauma
• Microscopically:
  • Hyperplastic superficial perivascular dermatitis
  • Mast cells, eosinophils, nonmetachromatic mononuclear cells
  • Perivascular inflammation may be involved especially in horses

Autoimmune reactions

Bullous pemphigoid

• Dogs and horses
• Involves oral cavity, mucocutaneous junctions, groin and axilla
• Subepidermal vesicles and bullae
• Antibodies bound to basement membrane
• Grossly:
  • Similar to Pemphigus vulgaris
• Microscopically:
  • Bullae containing fibrin, neutrophils or eosinophils
  • Basement membrane forms floor of bullae and roof is lined with basal cells
• Bullae may rupture -> ulcers

Dermatomyositis

• See also Canine dermatomyositis
• Affects puppies of collies and shetland sheepdogs from 8 weeks of age
• Lesions - vesiculating dermatitis
  • Face, lips, external ears, later distal extremities
• Microscopically: interface dermatitis and basal cell degeneration of epidermis and follicular wall, atrophy of follicles, epidermal vesicles and pustules, dermal scarring

Lupus erythematosus

• Systemic (SLE)
  • Multiple organs involved
  • Cats, dogs, horses
  • Immune dysregulation:
    • Damaged T-cell suppressor function, either primary deficiency or antibody mediated
    • Cytokine dysregulation
    • Resulting B-cell hyperactivity -> antibodies to self antigens -> antigen-antibody complexes deposited in various tissues -> Type III hypersensitivity
  • Lesions localised or generalised
  • Erythema, alopecia, depigmentation, crusting and scaling, ulceration
  • Microscopically: lymphohistiocytic interface dermatitis, thickened basement membrane, vasculitis, subepidermal vesicles, basal cell degeneration
• Discoid
  • Milder variant of systemic
  • Depigmentation, erythema, scaling, erosions, ulceration, crusting
  • Usually involves nasal planum, dorsum of muzzle, occasionally pinnae, lips, oral mucosa or periocular area
  • Microscopically: lichenoid interface dermatitis, often with lymphocytes, plasma cells, basal cell degeneration, loss of pigment