Human leukocyte antigen (HLA) DR4 occurs in about 70% of patients with rheumatoid ahritis HLA DR4 encoded in the major histocompatibility complex (MHC) region on
**Core Concept**
The question is testing the association between specific HLA alleles and rheumatoid arthritis (RA), highlighting the role of the major histocompatibility complex (MHC) in autoimmune disease susceptibility. HLA-DR4 is a particular allele within the HLA-DRB1 gene, which plays a crucial role in antigen presentation and immune system regulation.
**Why the Correct Answer is Right**
HLA-DR4 is strongly associated with rheumatoid arthritis, with around 70% of patients carrying this allele. The HLA-DRB1 gene, located in the MHC region on chromosome 6, encodes the HLA-DR4 protein. This protein is expressed on the surface of antigen-presenting cells (APCs) and plays a key role in recognizing and presenting peptide fragments to T-cells. The specific amino acid sequence of HLA-DR4 allows it to bind and present certain epitopes, including autoantigens, which may trigger an autoimmune response in susceptible individuals.
**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because the question specifically asks about the location of the HLA-DR4 gene, not its function or association with RA.
**Option B:** Incorrect because HLA-DR4 is not encoded in the HLA-DQ region, but rather in the HLA-DR region.
**Option C:** Incorrect because HLA-DR4 is not located on chromosome 1, but rather on chromosome 6.
**Clinical Pearl / High-Yield Fact**
The MHC region on chromosome 6 is a hot spot for genetic associations with autoimmune diseases, including rheumatoid arthritis, type 1 diabetes, and multiple sclerosis. Understanding the role of HLA alleles in disease susceptibility can inform diagnosis and treatment strategies.
**Correct Answer: D.** Chromosome 6.