Swelling is an increase in the size or a change in the shape of an area of the body. Swelling can be caused by collection of body fluid, tissue growth, or abnormal movement or position of tissue.

Most people will have swelling at some time. When it is hot and you have stood or sat in the same position for a long time, you might notice swelling in your feet and ankles. Staying in one position for any length of time increases the risk that the lower legs, feet, or hands will swell because body fluid will normally move down a limb from the effects of gravity. Swelling can also be caused by heat-related problems, such as heat edema from working or being active in a hot environment.

Body fluid can collect in different tissue spaces of the body (localized) or can affect the whole body (generalized). Causes of localized swelling include:

• Injury to a specific body area. Bruising (contusion) from an injury is caused by tears in the small blood vessels under the skin. Bleeding can also affect the joint (hemarthrosis) or the area that cushions and lubricates the joint (traumatic bursitis). Swelling can affect just one area or may involve large sections of the body, such as swelling that occurs following a motor vehicle accident.
• Infection, which can occur in a joint or under the skin. An abscess is a pocket of pus that forms at the site of infected tissue. Cellulitis is a skin infection that can cause mild or severe swelling.
• Burns, which can cause swelling at the site of the burn or in a larger area around the burn.
• Inflammation that occurs when tissue is irritated by overuse or repeated motion.
  • Swelling of the tendon and swelling caused by a series of small tears around a tendon (tendinosis) can occur together or separately.
  • Swelling of the sac that cushions and lubricates the joint (bursitis) can be caused by prolonged or repeated pressure or by activities that require repeated twisting or rapid joint movements.
• Insect bites or stings. Most insect bites or stings cause a small amount of redness or swelling. Some people have an allergic reaction to a bite or sting and develop a lot of swelling, redness, and itching.
• Other causes, such as swelling related to a saclike structure with clear fluid, blood, or pus (cyst) or a swollen organ, such as a salivary gland. For more information, see the topic Swollen Glands.

Causes of generalized swelling include:

• Allergic reaction. Sudden swelling of the hands and face may be a severe allergic reaction and needs immediate medical evaluation.
• Autoimmune diseases, such as lupus, rheumatoid arthritis, and scleroderma. These diseases can cause swelling when the body produces antibodies and other cells that attack and destroy tissues in the body.
• Medications. Some medications change how body fluids circulate, causing swelling. Swelling may also occur as an allergic reaction to a medication.
• Circulation problems related to certain medical conditions, such as peripheral arterial disease, heart failure, diabetes, or kidney disease. Thrombophlebitis causes swelling of an extremity when a blood clot interrupts blood flow in a vein in the arm or leg.
• Fluid that accumulates in the abdomen (ascites) because of other problems, such as malnutrition, obesity, cirrhosis, or liver disease.