HLA B 27 is not seen in
**Core Concept**
HLA B27 is a human leukocyte antigen (HLA) subtype that plays a crucial role in the immune system, particularly in the context of autoimmune diseases. It is a class I MHC molecule expressed on the surface of cells, responsible for presenting peptides to CD8+ T cells.
**Why the Correct Answer is Right**
HLA B27 is associated with several autoimmune conditions, including ankylosing spondylitis, reactive arthritis, and psoriatic arthritis. The exact mechanism of how HLA B27 contributes to these diseases is not fully understood, but it is thought to involve molecular mimicry and the presentation of self-peptides to autoreactive T cells. This leads to an inappropriate immune response and inflammation in affected joints and tissues.
**Why Each Wrong Option is Incorrect**
* **Option A:** This is incorrect because HLA B27 is indeed seen in patients with ankylosing spondylitis, a condition characterized by chronic inflammation of the spine and other joints.
* **Option B:** This is incorrect because HLA B27 is also associated with reactive arthritis, a type of arthritis that occurs in response to infection with certain bacteria, such as Chlamydia trachomatis.
* **Option D:** This is incorrect because HLA B27 has been linked to other autoimmune conditions, including psoriatic arthritis and uveitis.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the presence of HLA B27 is not diagnostic of any specific condition, but rather increases the likelihood of developing certain autoimmune diseases. A positive HLA B27 test result should be interpreted in the context of clinical presentation and other diagnostic findings.
**Correct Answer:** D. HLA B27 is not seen in patients with multiple sclerosis, a chronic autoimmune disease affecting the central nervous system.