Ig active in type 1 hypersensitivity.

**Core Concept**
Type 1 hypersensitivity reactions involve the release of pre-formed antibodies, specifically IgE, which bind to high-affinity receptors on mast cells and basophils. This binding leads to cell activation and the subsequent release of histamine and other mediators, resulting in clinical manifestations such as allergic rhinitis, asthma, and anaphylaxis.

**Why the Correct Answer is Right**
The correct answer is IgE because it is the primary antibody responsible for triggering type 1 hypersensitivity reactions. IgE binds to the high-affinity IgE receptor (FcεRI) on the surface of mast cells and basophils, which are then activated and release histamine and other chemical mediators. This leads to increased vascular permeability, smooth muscle contraction, and mucous secretion, resulting in the characteristic symptoms of type 1 hypersensitivity reactions.

**Why Each Wrong Option is Incorrect**
**Option A:** IgG is a type of antibody that plays a key role in secondary immune responses and is involved in type II and type III hypersensitivity reactions, but it is not the primary antibody responsible for type 1 hypersensitivity reactions.

**Option B:** IgM is another type of antibody that is involved in primary immune responses and is the first antibody to be produced in response to an antigen. However, it is not typically associated with type 1 hypersensitivity reactions.

**Option C:** IgA is a type of antibody that plays a key role in mucosal immunity and is found in high concentrations in secretions such as saliva, tears, and breast milk. While it can be involved in some allergic reactions, it is not the primary antibody responsible for type 1 hypersensitivity reactions.

**Option D:** IgD is a type of antibody that is primarily found on the surface of mature B cells and plays a role in initiating immune responses. However, it is not typically associated with type 1 hypersensitivity reactions.

**Clinical Pearl / High-Yield Fact**
IgE is often referred to as the "allergy antibody" because of its central role in type 1 hypersensitivity reactions. Remember the acronym "HEAT" to associate IgE with type 1 hypersensitivity: H (histamine release), E (eosinophilic response), A (anaphylaxis), and T (type 1 hypersensitivity).

**Correct Answer:** A. IgE.