**Core Concept**
IL-2, or Interleukin-2, is a cytokine that plays a crucial role in the immune response, particularly in the activation and proliferation of T lymphocytes. It is a key regulator of the adaptive immune system, mediating cell-mediated immunity.

**Why the Correct Answer is Right**
IL-2 is primarily produced by T helper 1 (Th1) cells, particularly after antigen presentation and activation. This process involves the recognition of antigens by T cell receptors, which triggers the production of IL-2 through the activation of transcription factors such as NF-κB and AP-1. IL-2 then binds to its receptor on the surface of T cells, stimulating their proliferation and differentiation into effector cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Dendritic cells can produce various cytokines, but they are not the primary source of IL-2.
**Option B:** Macrophages can produce a wide range of cytokines, but IL-2 production is not their primary function.
**Option C:** B cells can produce some cytokines, but IL-2 production is not their primary role.

**Clinical Pearl / High-Yield Fact**
IL-2 is often used as an immunotherapy agent to stimulate the immune response in cancer patients, particularly in the treatment of melanoma and renal cell carcinoma.

**Correct Answer:** C. T helper 1 (Th1) cells