Skin Immunologic - Pathology

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

Mediated by pharmacologically active substances from mast cells and basophils
- Due to antigen-antibody (usually IgE) binding to receptors on those cells
Substances include histamine, serotonin, leukotriens, prostaglandins
Can be systemic or local
Skin becomes pruritic, raised erythematous borders of wheals
Immediate reaction
Includes:
- Atopic dermatitis
- Urticaria
- Angioedema
- Fly bite hypersensitivity
- Gastrointestinal parasites
- Food
Microscopically:
- Capillary dilation, oedema, mast cell degranulation, eosinophil infiltration

Cytotoxic reaction
IgG and IgM interaction with antigens bound to cellular membranes
Often involves complement fixation -> cellular damage
In skin - circulating antibody to proteins of desmosomes in intercellular areas along basement membranes at epidermal-dermal junction
Includes:
- Pemphigus
- Bullous pemphigoid

Complement fixing immune complexes
IgG or IgM
Complexes deposit in tissue -> fix complement -> cytokines and othe factors attrack neutrophils -> release lysosomal enzymes, activation of complement and coagulation, platelet aggregation -> tissue damage
Immune complex vasculitis -> purpura haemorrhagica
Includes:
- Systemic lupus erythematosus
- Canine dermatomyositis

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

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

Usually involves Type I reaction and Type IV reaction due to salivary antigens of Culicoides sp.
May be seasonally recurring or continuous depending on climate
Grossly:
- Papules, crusts, alopecia, excoriations, lichenification
Microscopically:
- Superficial and dep perivascular dermatitis
- Many eosinophils
- Epidermal hyperplasia
- Hyperkeratosis
- Dermal fibrosis
- Possibly eosinophilic folliculitis, intraepidermal pustules and eosinophilic granulomas

Usually involves Type I reaction and Type IV reaction and cutaneous basophil hypersensitivity
Mainly dorsolumbosacral area involved, abdomen, caudomedial thighs, flanks, neck of cats
Secondary trauma is self-inflicted
Grossly:
- Papular dermatitis
- Secondary excoriations
- Alopecic nodule if chronic
Microscopically:
- Hyperplastic superficial perivascular dermatitis
- Oedema, mast cells, basophils, eosinophils, lymphocytes, histiocytes
- Fibropruritic nodules covered by hyperplastic epidermis

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

Blistering diseases
Autoantibodies bind to desmosomal proteins -> cell adhesion disrupted -> vesicles
Types:
- Pemphigus vulgaris
  - Rare, severe form
  - Cats and dogs
  - Vesicles or bullae -> erosions and ulcers
  - Involves oral cavity, mucocutaneous junctions and skin
  - Microscopically:
    - Suprabasilar vesiculation and row of basal cells adhering to basement membrane
    - Superficial perivascular dermatitis
- Pemphigus vegetans
  - Rare
  - Dogs
  - Involves the trunk
  - Mild version of P. vulgaris
  - Vesicopustular dermatitis + epidermal hyperplasia -> papillomatous formations
- Pemphigus foliaceus
  - More common
  - Cats, dogs, horses, goats
  - Localised lesions or generalised
  - Symmetrical vesicular or pustular dermatitis, crusts, scales, alopecia
  - Microscopically:
    - Acantholytic cells
    - Subcorneal or intragranular pustules
    - In third of equine or canine cases eosinophils are predominant
- Pemphigus erythematosus
  - Cats and dogs
  - Mild form of P. foliaceus
  - Usually involves only face and external ears