## **Core Concept**
The question tests understanding of Major Histocompatibility Complex (MHC) molecules, specifically Human Leukocyte Antigens (HLA) in different cell types. HLA antigens are crucial for the immune system to differentiate between self and non-self cells. Most nucleated cells express HLA class I antigens, while HLA class II antigens are primarily expressed on antigen-presenting cells (APCs) like dendritic cells, B lymphocytes, and macrophages.

## **Why the Correct Answer is Right**
The correct answer, **D. Red Blood Cells**, is right because mature red blood cells lack a nucleus and most organelles, including the machinery necessary for protein synthesis and expression of HLA antigens. While they do have a plasma membrane, the expression of HLA class I is very low or absent on mature red blood cells. This characteristic is significant in transfusion medicine, as it reduces the immunogenicity of red blood cells.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Platelets express HLA class I antigens on their surface. These antigens can stimulate an immune response if the platelets are recognized as foreign.
- **Option B:** Lymphocytes, including T cells and B cells, express HLA antigens. B cells can express both HLA class I and class II antigens, especially when activated.
- **Option C:** Endothelial cells, like most nucleated cells, express HLA class I antigens. They can also express HLA class II antigens under certain conditions, such as inflammation.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that while mature red blood cells lack HLA antigens, they can still cause hemolytic reactions if there is an ABO blood group incompatibility. This is because ABO blood group antigens are pre-formed on the surface of red blood cells and can react with pre-formed antibodies in the recipient's plasma.

## **Correct Answer: D. Red Blood Cells**