our immune system treats it as an invasive- rushing white blood cells to combat it as I understand it. Hence the blisters, for me it gets into my blood stream and I end up with what looks like and feels like a heat rash everywhere, takes some nasty steroids to clear it up. My mom would get huge blisters from it. Don't remember my dad ever having a reaction to it. One of my dogs was kind enough to share some with me about 10 years back, so it has been a while since I have had to deal with it.