Which condition is characterized by the body's immune system attacking the lining of joints?

The condition characterized by the body's immune system attacking the lining of joints is rheumatoid arthritis. This autoimmune disorder leads to chronic inflammation in the joints, which can result in damage to the cartilage and bone over time. The immune system mistakenly targets the synovium, a thin membrane that lines the joints, causing swelling, pain, and rigidity. This inflammation can extend beyond the joints, affecting other systems in the body as well.

In contrast, osteoarthritis is a degenerative joint disease caused primarily by wear and tear on the joints rather than an autoimmune response. Gout is a type of arthritis resulting from the accumulation of uric acid crystals in the joints, leading to sudden and severe pain. Osteopenia refers to lower than normal bone density, which is associated with an increased risk of fractures but does not involve joint inflammation or immune system activity.