**Core Concept**
Human Leukocyte Antigen (HLA) is a group of genes that play a crucial role in the immune system by coding for proteins responsible for the recognition of self and non-self cells. The HLA system is essential for the proper functioning of the immune response, particularly in the context of transplantation and autoimmunity.

**Why the Correct Answer is Right**
The correct answer is related to the fact that HLA molecules are present on the surface of all nucleated cells in the body, making them a crucial component of the immune system. The HLA molecules are involved in the presentation of antigens to T-cells, which helps in the activation of the immune response. The HLA system is also involved in the regulation of the immune response, and any abnormalities in the HLA system can lead to autoimmune diseases or transplant rejection.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify the correct location of HLA molecules. While HLA molecules are present on the surface of immune cells, they are not exclusive to these cells.
**Option B:** This option is incorrect because it is too broad and does not accurately describe the specific location of HLA molecules.
**Option C:** This option is incorrect because it is not a specific enough description of the location of HLA molecules.

**Clinical Pearl / High-Yield Fact**
The HLA system is essential for the proper functioning of the immune response, and any abnormalities in the HLA system can lead to autoimmune diseases or transplant rejection. Understanding the HLA system is crucial for the management of patients with autoimmune diseases and for the selection of donors and recipients in transplantation.

**Correct Answer: B. All nucleated cells in the body.**