A cold sore is a viral infection, like many other viral infections it doesn't require treatment, in 2-4 weeks the blisters go away. If you feel pain, you may apply antiviral medicine, but it's not always necessary. To boost your immune, you may take multi-vitamin, or simply Vitamin C, or plenty of fruits and vegetables, or Echinacea. Drink water, keep hydrated, sleep well, these are weapons to boost the immune as well. Drink green tea also helps.