The first change is swelling of the inner lining of the joints (Synovial membrane) with accumulation of white blood cells (neutrophils, lymphocytes and macrophages). The inflammation of synovial membrane may result in leakage and accumulation of synovial fluid in the joint space. In later stages, the synovial membrane thickens and projects into the joint cavity in the shape of long fingers. The thick, swollen, congested, synovial membrane creeps over and under the articular cartilage (pannus formation). This pannus gradually erodes the articular cartilage and the underlying bone over a period of time, leading to reduction in joint space and loss of free movement at the joint.

As the disease progresses, the muscles atrophies and the joint becomes fixed in odd positions (flexion contractures), essentially because the patient tends to keep his limb bent in a position of maximum relief. Later, the pannus may meet across the joint and get fibrosed (hard tissue). This leads to fusion of the joint known as ankylosis.

Along with these changes, small rounded nodules called rheumatoid nodules may also form under the skin and other places like the lungs, heart and eyes.