Never pop poison ivy blisters! Although they may be painful, an open blister can easily become infected and lead to blood poisoning. The blisters form as part of your body's immune response to poison ivy and oak and are part of the healing process.
The fluid from the blisters do not contain urushiol, the oil that causes poison ivy or oak rash, so scratching or breaking a blister will not cause the rash to spread. If your rash is continuing to spread, you are coming in contact with the plant's oil either through direct or secondary contact. Washing with a product like Tecnu Original or Tecnu Extreme will remove any urushiol oil, the rash-causing oil, that has not bonded to the skin.
If the blisters do break, cover the area loosely with a sterile bandage protecting the wound from bacteria. In severe cases, you should always contact your doctor.
