## **Core Concept**
Allergic hypersensitivity reactions are overreactions of the immune system to foreign substances, such as allergens. These reactions involve various immune cells and molecules, leading to tissue damage and clinical symptoms. The primary immune cells responsible for mediating allergic hypersensitivity are **Immunoglobulin E (IgE) antibodies** and the cells that interact with them.

## **Why the Correct Answer is Right**
The correct answer, **Immunoglobulin E (IgE)**, is right because IgE antibodies play a pivotal role in mediating allergic hypersensitivity reactions. When an allergen enters the body, it is recognized by **B cells**, which then produce IgE antibodies. These IgE antibodies bind to the surface of **mast cells** and **basophils**. Upon subsequent exposure to the same allergen, the allergen binds to the IgE antibodies on the surface of these cells, causing them to degranulate and release various **mediators**, such as histamine, leukotrienes, and prostaglandins. These mediators lead to the symptoms associated with allergic reactions, such as vasodilation, smooth muscle contraction, and increased vascular permeability.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While T cells, particularly **Th2 cells**, play a role in the regulation of IgE synthesis and in the mediation of allergic responses by promoting the production of IgE and the activation of eosinophils, they are not the direct mediators of allergic hypersensitivity reactions.
- **Option B:** Although eosinophils are involved in allergic reactions, especially in the late phase and in diseases like asthma and atopic dermatitis, they are not the primary mediators of allergic hypersensitivity.
- **Option C:** This option might refer to other types of immunoglobulins (like IgG, IgA, IgM), which are not primarily involved in mediating allergic hypersensitivity reactions.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **Type I hypersensitivity reactions**, which include allergic reactions and anaphylaxis, are IgE-mediated. This is in contrast to other types of hypersensitivity reactions (Type II, III, and IV), which are mediated by different immune mechanisms. Clinically, this is important because treatments and preventive measures can be targeted based on the underlying immune mechanism.

## **Correct Answer:** B. Immunoglobulin E (IgE).