RA antibody causes which type of hypersensitivity?
**Core Concept**
The question tests the understanding of immunological principles, specifically the type of hypersensitivity reaction mediated by Rheumatoid Arthritis (RA) antibodies. RA antibodies are associated with **autoimmune disorders** and play a role in the pathogenesis of rheumatoid arthritis.
**Why the Correct Answer is Right**
RA antibodies, also known as rheumatoid factors, are **autoantibodies** that target the Fc region of IgG antibodies, leading to the formation of immune complexes. These immune complexes can deposit in tissues, triggering an inflammatory response mediated by **complement activation** and **neutrophil infiltration**, characteristic of a **type III hypersensitivity reaction**.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because type I hypersensitivity involves IgE antibodies and is associated with allergic reactions, not autoimmune responses like RA.
**Option B:** Incorrect as type II hypersensitivity involves direct antibody-mediated cytotoxicity, which is not the primary mechanism of RA antibodies.
**Clinical Pearl / High-Yield Fact**
Remember that type III hypersensitivity reactions, like those seen in RA, involve immune complex deposition and can lead to tissue damage in various organs, including the joints, skin, and kidneys.
**Correct Answer:** D. Type III hypersensitivity.