The applicability of FAR 121.377 is not eliminated by a person's performing a mix of work such as you describe. Such an interpretation would defeat the purpose of the regulation which is to insure that person's performing part 121 maintenance or preventive maintenance are provided relief from duty.
The cause of rheumatoid arthritis is unknown, although it appears to be an autoimmune disease. When the body's immune system does not operate as it should, white blood cells that normally attack bacteria or viruses attack healthy tissue instead â in this case, the synovium, or joint tissue.
This is not a direct indication as to how commonly these diseases are the actual cause of Rheumatoid factor positive, but gives a relative idea as to how frequent these diseases are seen overall. 1  3 diseases that are common. 2  2 diseases that are very rare. 3  8 diseases without any prevalence information. Causes of Rheumatoid factor positive that are common The following causes of Rheumatoid factor positive are diseases or conditions that affect more than 1 million people in the USA: Connective tissue diseases.