Which of the following cytokines is also known as the T-cell growth factor?

• A. IFN-γ
• B. IL-12
• C. IL-2
• D. IL-10

Answer: (C)

The key idea is that this cytokine directly drives T cell proliferation after activation. When T cells encounter an antigen, they produce this cytokine, which acts on the same cells and neighboring ones to promote clonal expansion. This autocrine and paracrine growth signal is what earned it the nickname T-cell growth factor. It signals through the IL-2 receptor, which is upregulated on activated T cells, including regulatory T cells, to bolster the T cell response.

The other cytokines have different roles: one primarily activates macrophages and supports antiviral responses, another promotes differentiation of naive T cells into a Th1 lineage and enhances IFN-γ production, and the last is anti-inflammatory, dampening immune activation. Thus, IL-2 is the cytokine best described as the T-cell growth factor.