This looks like severe dermatitis herpetiformis. I have celiacs disease (autoimmune disease) and when I eat gluten I get a much milder form of this on my skin. When it happens on my palms or the sole of my feet it looks like this but never this bad. Whoever took this picture is probably in pretty severe pain and unable to use the hand.

The cause of the sores is your immune system destroying cells in the lower portion of your skin. The debris from the dead cells then forms those white blisters and they migrate upwards to be expelled by the body. The action is similar to chickenpox/herpes/cold sores except in that case the dead cells are casualties of your body fighting off the virus. That is why it looks so similar.