## **Core Concept**
The question tests the knowledge of interleukins and their functions, specifically which one acts as T-cell growth factor. Interleukins are a group of cytokines that were initially found to be expressed by white blood cells (leukocytes). They play a crucial role in the body's immune response.

## **Why the Correct Answer is Right**
Interleukin-2 (IL-2) is known as T-cell growth factor because it is a type of cytokine signaling molecule in the immune system. IL-2 is crucial for the growth, proliferation, and survival of T cells. It is produced primarily by T cells and acts in both an autocrine (on the same cell) and paracrine (on nearby cells) manner to stimulate the growth and differentiation of T cells.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While IL-1 is an important cytokine involved in the inflammatory response and activation of T cells, it is not specifically known as T-cell growth factor.
- **Option B:** IL-6 is involved in inflammation and infection responses and can influence T cell differentiation but is not primarily known as T-cell growth factor.
- **Option D:** IL-12 is crucial for the differentiation of naive T cells into Th1 cells but is not identified as T-cell growth factor.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that IL-2 is not only crucial for T-cell growth and proliferation but also for the development of regulatory T cells, which are vital for maintaining immune tolerance. This makes IL-2 a significant target for immunosuppressive therapies in transplant medicine and autoimmune diseases.

## **Correct Answer:** .