When rheumatoid arthritis flares up, it makes joints feel stiff and achy. That discomfort may go away at times, but there may still be permanent damage. Eventually rheumatoid arthritis can harm joints so they don't work as well even when the disease itself is not active. How does joint damage occur, and how can it be prevented?
Doctors call the active periods of rheumatoid arthritis disease activity. During disease activity, infection-fighting cells (white blood cells) are mistakenly allowed into the joint. No one understands why this happens, but it's clear the infection fighters don't belong there.
Inside the joint, these cells produce chemicals that they usually use to kill invading microorganisms - only none are there. Instead, the chemicals damage the healthy joint tissue. During high levels of disease activity, you experience a flare - joints become swollen, stiff, and painful. You can also have low levels of disease activity that come and go without your feeling any symptoms.
http://www.webmd.com/rheumatoid-arthritis/guide/preventing-joint-damage-rheumatoid-arthritis