A blister is small, fluid-filled vesicle that forms between the upper layers of the skin due to a variety of factors. According to Lifespan, a blister that forms as part of a skin rash may be tiny, as small as the head of a pin, or much larger, up to one-half inch wide. Blisters associated with a skin rash are typically circular-shaped and filled with a serous fluid that may leak out, forming a yellow crust.

Contact Dermatitis

Contact dermatitis is a skin rash that forms when the skin comes into contact with substances that cause irritation or an allergic reaction. According to the Merck Manuals, the rash is red and itchy with clearly defined borders. Blisters may form along with the rash. Chemical irritants include nail polish removers and tanning agents used in shoe manufacture. Antibiotics and scents used in deodorants and hand lotions can cause an allergic contact dermatitis. Treatment includes removing the offending agent and avoiding the substance that causes the rash.

Poison Ivy

Poison ivy causes an allergic contact dermatitis. According to FamilyDoctor.org, poison ivy contains the oil urushiol, which bonds to the skin with contact. Poison ivy is found in most places in the United States, and 50 percent to 70 percent of people are sensitive to it. The rash appears in 24 to 48 hours, initially being red and swollen and then forming small blisters. The blisters are extremely itchy, but avoid scratching as this spreads the rash. Treatment includes removing the oil with a solvent, such as Technu, and oral and topical antihistamines to relieve itching.

Impetigo

Impetigo is a bacterial skin rash that's common in children but can also affect adults. According to MedlinePlus, impetigo is caused by staphylococcus or streptococcus bacteria. It occurs where there is a break in the skin, causing a rash of small blisters filled with a honey-colored fluid that break open to show a red base. Impetigo can occur anywhere on the body. Impetigo must be treated with topical or oral antibiotics.

Chickenpox and Shingles

Chickenpox is a childhood disease that causes an itchy rash of small spots that first develop on the abdomen or back. These spots form blisters as the disease progresses. Chicken pox is caused by the varicella-zoster virus. Later in life, you can develop shingles if you've had chickenpox as a child. According to the American Academy of Dermatology, shingles is the reactivation of the varicella-zoster virus. The rash typically forms first on the buttocks or trunk, following a nerve path. Initially, there's a burning sensation and then small blisters appear that last for two to three weeks. Vaccinations are available for chickenpox and shingles.

Other Causes

Athlete's foot, a fungal infection, can infect the feet and cause an itchy rash with small blisters on the feet and toes. Dermatitis herpetiformis is an extremely itchy rash of blisters and bumps that may be associated with celiac disease. Bullous pemphigoid is a rare condition that usually affects elderly people and causes large blisters to form on the body.