**Core Concept**
Type I hypersensitivity, also known as immediate hypersensitivity, is a type of allergic reaction mediated by IgE antibodies. This reaction occurs when an allergen binds to the IgE antibodies on the surface of mast cells and basophils, leading to the release of histamine and other chemical mediators, resulting in allergic symptoms.

**Why the Correct Answer is Right**
The correct answer is mast cells. Mast cells are the primary effector cells in type I hypersensitivity reactions. When an allergen binds to the IgE antibodies on the surface of mast cells, it triggers the release of histamine, leukotrienes, and other chemical mediators. These mediators cause blood vessels to dilate, smooth muscle to contract, and mucous glands to secrete, resulting in symptoms such as itching, swelling, and increased mucus production. Basophils also play a role in type I hypersensitivity, but their role is less prominent than that of mast cells.

**Why Each Wrong Option is Incorrect**
**Option A:** Basophils are involved in type I hypersensitivity, but they are not the primary effector cells like mast cells. Basophils are more involved in chronic inflammation and are not as crucial in acute allergic reactions.

**Option B:** Neutrophils are involved in various types of inflammation, but they are not typically associated with type I hypersensitivity reactions.

**Option C:** Lymphocytes are involved in adaptive immune responses, but they are not directly involved in the effector phase of type I hypersensitivity reactions.

**Clinical Pearl / High-Yield Fact**
Mast cells are responsible for the release of histamine and other chemical mediators in type I hypersensitivity reactions, which can lead to symptoms such as anaphylaxis, bronchospasm, and increased mucus production.

**Correct Answer:** A. Mast cells.