A vesicle is small -- it may be as tiny as the top of a pin or up to 5 or 10 millimeters wide.

In many cases, vesicles break easily and release their fluid onto the skin. When this fluid dries, yellow crusts may remain on the skin surface.

Many diseases and conditions can cause vesicles. Some common examples include:

• Allergic reactions to drugs
• Atopic dermatitis (eczema )
• Blistering skin diseases including porphyria cutanea tarda and dermatitis herpetiformis
• Chicken pox
• Contact dermatitis (may be caused by poison ivy)
• Herpes simplex (cold sores, genital herpes)
• Herpes zoster (shingles)
• Impetigo

As a general rule, your doctor should examine any skin rashes, including vesicles.

Over-the-counter treatments are available for certain conditions that cause vesicles, including poison ivy and cold sores.

Call your doctor if you have any unexplained blisters on your skin.

Your doctor will look at your skin. Some vesicules can be diagnosed simply by how they look.

In many cases, however, additional tests are needed. The fluid inside a blister may be sent to a lab for closer examination. In particularly difficult cases, a skin biopsy may be needed to make or confirm a diagnosis.

Goldman L, Ausiello D. Cecil Textbook of Medicine. 22nd ed. Philadelphia, Pa: WB Saunders; 2004.

Habif TP. Clinical Dermatology. 4th ed. St. Louis, Mo: Mosby; 2004.