**Core Concept**
Contact dermatitis is a type of skin inflammation caused by exposure to an allergen or irritant. It involves a complex interplay of immune cells and mediators, leading to skin barrier disruption and clinical manifestations.

**Why the Correct Answer is Right**
The major role in contact dermatitis is played by T lymphocytes, specifically CD4+ T helper cells (Th1 and Th2). These cells recognize allergens presented by antigen-presenting cells (APCs) and orchestrate an immune response. Th1 cells promote cell-mediated immunity, while Th2 cells facilitate the production of IgE antibodies and the release of histamine and other mediators. This leads to increased vascular permeability, edema, and inflammation in the affected skin.

**Why Each Wrong Option is Incorrect**
* **Option A:** Macrophages play a role in antigen presentation and the release of pro-inflammatory cytokines, but they are not the primary cells involved in contact dermatitis.
* **Option B:** Neutrophils are involved in the inflammatory response, but their primary role is in bacterial and fungal infections, not contact dermatitis.
* **Option C:** Dendritic cells are APCs that present antigens to T cells, but they are not the major effector cells in contact dermatitis.

**Clinical Pearl / High-Yield Fact**
In contact dermatitis, the use of topical corticosteroids can help reduce inflammation and prevent further immune activation. It's essential to identify and avoid the offending allergen to prevent recurrence.

**Correct Answer:** C. T lymphocytes.