Rheumatoid arthritis affects the joints and is due to inflammation. This type of arthritis is systemic and chronic. The cause of rheumatoid arthritis is currently unknown though it is genetic and can be linked to a specific gene of which over 90% of rheumatoid arthritis suffers all have. It is speculated that RA may be linked to some infections such as parovirus, Epstein Barr virus and rubella but no direct linkage has been discovered.
The inflammation that causes arthritis is due to your immune system thinking that your joints have an infection or something that requires an immune response. Your white blood cells, T cells, fibroblasts, neutrophils and other immune cells will then attack the joint causing inflammation.