Your ankle will swell after a knee injury because the body is trying to get all the weight off the knee. It does this because it wants it to heal and so all the weight shifts to the ankle. Limping will also cause the ankle to have a lot more weight on it.
It's the normal inflammation process. Following injury to your tissues, increased movement of plasma and leukocytes from the blood into the injured tissues takes place. This movement is achieved via the chemically-induced dilation and increased permeability of blood vessels, which results in a net loss of blood plasma. The increased collection of fluid into the tissue causes it to swell.
Also, keep in mind that while some swelling can have very negative impact, that the swelling of tissue can actually help stop blood loss... As a survival reaction, this is usually considered the primary reason for our bodies to react in such a way...
When an injury in the knee or ankle occurs, the normal cells in the region multiply to provide extra cushion to the affected area. This is what actually causes the knee or ankle to appear swollen. This sudden increase in cells and the resulting swelling is termed as hyperplasia. In addition to that, there is a little internal bleeding as a direct result of the injury.
With regards to swelling, there are two different types of the state, namely false tumours and true tumours. The first category is inclusive of inflammatory swellings includes injuries like bruises, fractures, sprains, black eyes, infectious swellings such as boils and abscesses.
As for true tumours, these are the swellings that are developed from masses of tissues developing from already existing body cells. Their growth does not cease and instead continues. It is even true that some of these second-type tumours are benign and bring no real harm to the body.
