**Core Concept**
Regulatory T cells (Tregs) play a crucial role in maintaining immune homeostasis by suppressing excessive or auto-reactive immune responses. They are essential for preventing autoimmune diseases and maintaining tolerance to self-antigens.

**Why the Correct Answer is Right**
Regulatory T cells function through several mechanisms, including the production of anti-inflammatory cytokines such as IL-10 and TGF-β, which inhibit the activity of effector T cells. They also express the surface molecule CTLA-4, which competes with CD28 for binding to B7 molecules on antigen-presenting cells, thereby reducing T cell activation. Additionally, Tregs can directly suppress effector T cell function through cell-cell contact and the release of immunosuppressive molecules such as adenosine.

**Why Each Wrong Option is Incorrect**
* **Option A:** Macrophages are involved in phagocytosis and antigen presentation, but they do not primarily function in suppressing the immune response.
* **Option B:** Dendritic cells are professional antigen-presenting cells that activate T cells, rather than suppressing them.
* **Option C:** B cells produce antibodies and can participate in antibody-dependent cellular cytotoxicity (ADCC), but they do not primarily function in suppressing the immune response.

**Clinical Pearl / High-Yield Fact**
Regulatory T cells are essential for preventing autoimmune diseases, such as type 1 diabetes and rheumatoid arthritis, and their dysfunction has been implicated in the pathogenesis of various autoimmune disorders.

**Correct Answer:** C. Regulatory T cells (Tregs)