Swelling is defined as an increase in size or change in the shape of body parts. The reasons behind this swelling can be varied - abnormal tissue growth, collection of bodily fluids or even an out of the ordinary positioning of tissue.

A majority of people have experienced swellings at one point of time or the other in their lives. Sometimes the reasons can be environmental, like when it's too hot outside, or you've remained in the same position for a long period of time. Remaining in one position results in swelling of the hands, legs etc because movement of body fluid becomes restricted.

The collection of the body fluid can happen in two different types of tissue spaces - Generalized and Localized.

Causes of swelling:

Allergic Reactions causes sudden swelling on face and hands, and in some cases, to the entire body. The allergy can be to almost anything - food, medicines, toxin in the air etc. Medicines especially can change the manner in which body fluids circulate within the body. Severe allergic reactions are called anaphylaxis and they require immediate medical attention.
Autoimmune Diseases such as lupus, scleroderma and arthritis can cause the body to swell. In these conditions, the body starts producing antibodies which end up destroying tissues in the body.
Circulatory Problems that arise due to medical complications like diabetes, peripheral arterial disease, or even kidney disease can cause the entire body to swell.

Following are the causes of localized swelling:

Bruising, also called hematoma, happens when the blood vessels under one's skin tears up. The swelling can be in just one particular area or can be spread over bigger sections of the body.
Burns can cause instant swelling. Whichever part of the skin is exposed to fire can cause swelling in that area, as well as the area surrounding it at
Infections can occur both on the skin and under it. Cellulitie causes mild to severe swelling, depending on whether the infection is mild or not.
Sometimes, the tendon can also swell as a result of numerous series of small tears around it, which can occur simultaneously or over a period of time.
The bursitis is the sac which cushions and lubricates the joints in our body, and it can swell when exposed to strenuous physical activities consistently over a long period of time, especially those activities which require quick movements of the joints.
Even external environmental phenomena like insect bites or stings from bees or wasps can cause swellings. However, most of the times these bites are not dangerous; they are characterized by a red swelling which fades over the passage of time. Some children might experience an allergic reaction to said stings or bites, due to which the affected area might swell excessively and itch continuously..
Other causes, such as swelling related to a sac-shaped structure with clear fluid, blood, or pus (cyst)