Pemphigus foliaceus is the most common autoimmune skin disease in horses and is associated with the production of autoantibodies directed against surface proteins of the keratinocyte. Pemphigus vulgaris is a rare autoimmune skin disease in horses. Systemic lupus erythematosus and cutaneous lupus erythematosus are recognized in horses and both are rare. Bullous pemphigoid is a rare autoimmune disease in horses caused by immunologic attack of the basement membrane zone by autoantibodies. Erythema muliforme is an immunologic reaction in the skin in which keratinocyte cell death is the prominent change seen on biopsy. Purpura hemorrhagica is thought analogous to nonthrombocytopenic purpura in humans.