**Core Concept:** The Human Leukocyte Antigen (HLA) complex is a group of genes that encode proteins involved in immune system function, particularly in cell-mediated immunity and transplantation. HLA molecules are located on the surface of human cells and help identify foreign substances, such as pathogens and transplanted tissues.

**Why the Correct Answer is Right:** The HLA complex is located on chromosome 6 in humans, specifically on the short arm (p) of chromosome 6. Each HLA gene is inherited from one parent, making them highly diverse and essential for the immune system's ability to recognize and respond to foreign substances.

**Why Each Wrong Option is Incorrect:**

A. Chromosome 4 (Option A is incorrect because the HLA complex is actually located on chromosome 6, as explained above.)
B. Chromosome 2 (Option B is incorrect, as the HLA complex is located on chromosome 6, not 2.)
C. Chromosome 14 (Option C is incorrect, as the HLA complex is located on chromosome 6, not 14.)
D. Chromosome 17 (Option D is incorrect, as the HLA complex is located on chromosome 6, not 17.)

**Clinical Pearl / High-Yield Fact:** Understanding the location of HLA complex is crucial for transplantation medicine. Donor and recipient HLA compatibility plays a major role in determining the success of organ or tissue transplantation. A higher degree of HLA compatibility increases the chances of a successful transplantation and reduced risk of rejection and complications.