Which cytokine is primarily produced by activated T cells to promote expansion of the T cell population?

• A. IL-15
• B. IL-7
• C. IL-12
• D. IL-2

Answer: (D)

Activated T cells produce interleukin-2, which acts as a growth factor to drive their own clonal expansion. When a T cell recognizes antigen, it upregulates IL-2 and the high-affinity IL-2 receptor (which includes CD25). IL-2 then binds to this receptor on the same cell and neighboring T cells, triggering JAK-STAT signaling that pushes cells through the cell cycle and expands the population. While other cytokines like IL-7, IL-12, and IL-15 have important roles in T cell survival, differentiation, and memory/NK cell maintenance, they are not the primary drivers of T cell proliferation after activation. Therefore, IL-2 is the cytokine most directly responsible for expanding the activated T cell population.