**Core Concept**
Type I hypersensitivity reactions, also known as immediate hypersensitivity reactions, are an allergic response that occurs within minutes of exposure to an allergen. This reaction is characterized by the release of histamine and other chemical mediators from mast cells and basophils, leading to symptoms such as hives, itching, swelling, and difficulty breathing.

**Why the Correct Answer is Right**
The correct answer is D. IgE because it is the primary immunoglobulin responsible for triggering Type I hypersensitivity reactions. IgE antibodies bind to allergens and cross-link with IgE receptors on the surface of mast cells and basophils, leading to the activation of these cells and the release of histamine and other chemical mediators. This process is facilitated by the high-affinity IgE receptor (FcεRI) on the surface of mast cells and basophils. The binding of IgE to its receptor on these cells triggers a cascade of intracellular signaling events that ultimately lead to the release of histamine and other chemical mediators.

**Why Each Wrong Option is Incorrect**
**Option A:** IgG is not primarily involved in Type I hypersensitivity reactions. IgG antibodies are responsible for secondary immune responses and can activate the complement system, but they are not the primary mediators of Type I hypersensitivity reactions.
**Option B:** IgM is also not primarily involved in Type I hypersensitivity reactions. IgM antibodies are responsible for primary immune responses and can activate the complement system, but they are not the primary mediators of Type I hypersensitivity reactions.
**Option C:** IgD is not primarily involved in Type I hypersensitivity reactions. IgD antibodies are present on the surface of mature B cells and can participate in the activation of these cells, but they are not the primary mediators of Type I hypersensitivity reactions.

**Clinical Pearl / High-Yield Fact**
It's worth noting that Type I hypersensitivity reactions can be prevented or treated with medications such as antihistamines, corticosteroids, and epinephrine, which can help to alleviate symptoms and prevent further allergic reactions.

✓ Correct Answer: D. IgE. IgE is the primary immunoglobulin responsible for triggering Type I hypersensitivity reactions.