Which class does rheumatoid factor most commonly belong to?

Rheumatoid factor is primarily an antibody that belongs to the IgM class. It is an autoantibody that targets the Fc portion of IgG antibodies. The presence of rheumatoid factor is commonly associated with rheumatoid arthritis and can be used as a marker for the disease.

The IgM class of antibodies is the first type produced in response to an infection or an autoimmune condition. Its pentameric structure allows it to efficiently bind to antigens, making it a key player in immune responses. In the case of rheumatoid arthritis, the elevation of IgM rheumatoid factor levels signifies the body's immune system is mistakenly attacking its own tissues, particularly the joints.

Understanding the classification of rheumatoid factor as IgM helps in diagnosing autoimmune diseases and monitoring their progression. In contrast, other immunoglobulin classes like IgE, IgG, and IgA serve different roles in immune protection and response, making them less relevant in the context of rheumatoid factor.