**Core Concept**
Type I hypersensitivity, also known as immediate hypersensitivity or anaphylaxis, is a type of immune response characterized by the release of mediators from mast cells and basophils, leading to allergic reactions. This reaction is initiated by the binding of an allergen to a specific antibody.

**Why the Correct Answer is Right**
In type I hypersensitivity, the primary mediator is IgE (immunoglobulin E), which plays a central role in triggering allergic reactions. IgE antibodies bind to allergens, leading to cross-linking of high-affinity IgE receptors on the surface of mast cells and basophils. This cross-linking causes the release of histamine and other mediators, resulting in allergic symptoms such as increased vascular permeability, smooth muscle contraction, and mucus production. The release of histamine from mast cells and basophils leads to the characteristic symptoms of type I hypersensitivity reactions.

**Why Each Wrong Option is Incorrect**
**Option B:** IgG is the most common type of antibody and plays a crucial role in secondary immune responses, but it is not the primary mediator in type I hypersensitivity reactions.
**Option C:** IgM is the first antibody produced in response to an infection and is primarily involved in the initial immune response, but it is not the mediator responsible for type I hypersensitivity reactions.
**Option D:** There is no such antibody as IgC; this option is likely a distractor.

**Clinical Pearl / High-Yield Fact**
IgE antibodies are responsible for triggering allergic reactions, and the release of histamine from mast cells and basophils is the primary mechanism behind these reactions. Understanding the role of IgE in type I hypersensitivity is crucial for diagnosing and managing allergic conditions.

**✓ Correct Answer: A. IgE**