There are many types of dermatitis, but one of the most painfully irritating is Rhus dermatitis. Rhus dermatitis, or poison ivy rash, is an allergic reaction to the plant Toxicodendron radicans (poison ivy). The reaction is caused by the plant’s production of urushiol. Urushiol causes a severe itching reaction when most people touch the plant. The rash can spread virulently, however there are many poison ivy remedies available.