A Level Biology Practice Exam 2025 – All-in-One Study Guide to Excel in Your A Levels!

The correct answer is T helper cells, which play a crucial role in the immune system by producing interleukins. Interleukins are a group of cytokines that are important for cell signaling in the immune response. T helper cells are activated by antigen-presenting cells and, upon activation, release various interleukins that stimulate the differentiation and proliferation of other immune cells, including B cells and cytotoxic T cells. This coordination is essential for mounting an effective adaptive immune response.

Other cells, such as B cells, natural killer cells, and cytotoxic T cells, have important functions in the immune system, but they do not primarily produce interleukins. While B cells can secrete antibodies in response to interleukins, they do not synthesize these signaling molecules themselves. Natural killer cells and cytotoxic T cells are involved in directly killing infected or cancerous cells but do not have a primary role in interleukin production either.