Those symptoms would include stiffness in the morning that is prolonged. Other symptoms would be persistent swelling of joints or persistent aching in joints. One hard and fast rule is unusual symptoms and persistent symptoms. Other symptoms would be fatigue, weight loss, fevers. In most cases it can, or will, remit.
Most cases are very treatable.
Children can also develop a temporary or transient arthritis after a viral illness. This is not a serious problem and will last only a week or two. In terms of the immune system, that is a very complicated question. The best place would be to go to a reference book. The mechanics of the immune system are very complicated and is better found in a reference book.