Rheumatoid factor is mainly-
**Core Concept**
Rheumatoid factor (RF) is an antibody that plays a crucial role in the pathogenesis of rheumatoid arthritis (RA). RF is primarily an IgM antibody directed against the Fc portion of IgG antibodies.
**Why the Correct Answer is Right**
The correct answer is IgM because RF is mainly composed of IgM antibodies. The production of RF is driven by B-cell activation and the subsequent class-switching to IgM. RF is not just a passive marker of disease activity but actively contributes to the inflammatory process by activating complement and forming immune complexes, which deposit in the joints and lead to inflammation.
**Why Each Wrong Option is Incorrect**
* **Option A:** IgG is incorrect because while IgG antibodies can also be involved in the production of RF, the primary RF is IgM.
* **Option B:** IgA is incorrect because IgA is not the main type of RF; it is produced in smaller amounts and is not as clinically relevant.
* **Option C:** IgE is incorrect because IgE is primarily involved in allergic reactions and is not associated with RF production.
**Clinical Pearl / High-Yield Fact**
RF is not specific to RA and can be found in other autoimmune diseases, such as Sjögren's syndrome and lupus, as well as in some chronic infections. Therefore, a positive RF test should always be interpreted in the context of clinical and laboratory findings.
**Correct Answer:** A. IgM