Inflammation is a response from your immune system in response to an irritant. For example, if you sprain your ankle, your immune system creates a protein called a Circulating Immune Complex (CIC for short). The CIC travels down to the injured ankle and causes pain and swelling.
The pain you feel is to inform you of the injury or damage. And the swelling is protective as it prevents you from moving it and causing more irritation. This is also your body's way of running to the problem with fresh blood, antibodies and vital cells in order to begin healing and repairing the damage.
Then what normally happens is our bodies produce proteolytic enzymes which counteract the inflammation, and things return to normal. That's why a sprained ankle as a young child heals within a few weeks at most, but can take six weeks or more for an adult of say 45.
The problem is, after around age 25, our production of these enzymes drops off almost completely so there is nothing to tell the body to stop the inflammation. These enzymes are also responsible for cleaning the blood, fighting off viral and bacterial infections and breaking down excess fibrin (scar tissue).
In addition to the decrease in production of proteolytic enzymes, there are also numerous other things that contribute to the build up of inflammation in our bodies.