## **Core Concept**
Interleukin-2 (IL-2) is a type of cytokine signaling molecule in the immune system. It is a key regulator of the immune response, particularly important for the growth, proliferation, and survival of T cells. IL-2 plays a crucial role in the body's natural response to microbial infection and in immune responses, including those against cancer.

## **Why the Correct Answer is Right**
The correct answer, T-helper cells (also known as CD4+ T cells), are a type of T cell that play an important role in the immune system by helping activate cytotoxic T cells and B cells, as well as activating other immune cells such as macrophages. T-helper cells are known to secrete IL-2, among other cytokines, which is essential for the proliferation of T cells.

## **Why Each Wrong Option is Incorrect**
- **Option A:** B cells are primarily involved in the production of antibodies and are not known for the secretion of IL-2 as a primary function. While B cells can interact with T cells and influence their function, they are not the primary source of IL-2.
- **Option B:** Cytotoxic T cells (CD8+ T cells) can respond to IL-2 but are not the primary source of IL-2 secretion. They are more involved in directly killing infected cells or producing chemical signals that activate other immune responses.
- **Option C:** Natural Killer (NK) cells are part of the innate immune system and can produce some cytokines but are not the primary source of IL-2.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that IL-2 is used therapeutically in the treatment of certain types of cancer, such as renal cell carcinoma and melanoma. It works by enhancing the body's immune response against cancer cells. Remembering the role of IL-2 in immunotherapy can be crucial for understanding its clinical applications.

## **Correct Answer:** D. T-helper cells.