Final answer:
Activated APCs secrete Interleukin-2 (IL-2), which is a cytokine involved in the stimulation and proliferation of T cells during an immune response.
Step-by-step explanation:
Upon activation of antigen-presenting cells (APCs) via the 2 signals, it secretes Interleukin-2 (IL-2). This is not explicitly mentioned in the provided references, but based on the fact that APCs play a crucial role in activating T cells, which then secrete IL-2 to further stimulate their own growth and activation as well as the growth and activation of other immune cells such as B cells and cytotoxic T cells. IL-2 is a type of cytokine that is essential for the growth, proliferation, and differentiation of T cells after they have been stimulated by antigens presented by APCs. This forms a part of a larger immune response to infections or cancer cells.