This is a good question, and the answer is still the subject of research. In some instances swelling comes on immediately after a knee injury, in which case it is probably caused by trauma to other nearby tissues, rather than to the knee cartilage itself. However, in many cases a person with a damaged meniscus can actually walk on it immediately following the injury, with the swelling only occurring hours or even days later. In this case, the culprit appears to be proteoglycans, which are large molecules that are part of the cartilage tissue. When the cells and intercellular matrix of the cartilage are disrupted by an injury, the proteoglycans can leak into nearby tissues, causing the release of immune system molecules know as interleukins. The interleukins cause the swelling by increasing the permeability of the surrounding tissues, which is a common part of the immune response.