These are fluid-filled bumps on the skin. Many different things may cause blisters. Some of the major causes are burns, reactions to insect bites, chronic rubbing or trauma to the skin, and rashes from viral illnesses, such as chicken pox. Blisters, themselves, are usually filled with sterile fluid although they can arise as a reaction from a skin infection.
Most blisters cause no pain unless they rupture, exposing tender skin underneath.
The itching often associated with these skin lesions is usually not really from the blisters themselves, but from the irritated skin immediately surrounding the blister. Once the blister opens on its own, put antibiotic ointment on it until it is healed. If it is practical, try to protect the blister with an adhesive bandage. Watch this if baby's meals end with thrown food. Learn about a potentially lifesaving new vaccine.