**Core Concept**
Plasma cells are specialized B lymphocytes responsible for producing and secreting large amounts of antibodies, including IgE, a key mediator of allergic reactions. IgE plays a crucial role in triggering type I hypersensitivity reactions, which involve the release of histamine and other chemical mediators from mast cells and basophils.

**Why the Correct Answer is Right**
Plasma cells are the primary source of IgE antibodies in humans. They are activated B cells that undergo class switching to produce IgE, which is then secreted into the bloodstream. IgE antibodies bind to high-affinity IgE receptors on the surface of mast cells and basophils, leading to the cross-linking of these receptors and the subsequent release of histamine and other mediators. This process is central to the pathophysiology of allergic diseases, such as asthma and anaphylaxis.

**Why Each Wrong Option is Incorrect**
**Option A:** Mast cells are the primary target of IgE antibodies, but they are not responsible for secreting IgE. Instead, they play a key role in the allergic response by releasing histamine and other chemical mediators.
**Option B:** Basophils also express IgE receptors and are involved in the allergic response, but they are not the primary source of IgE antibodies.
**Option C:** Eosinophils are another type of white blood cell involved in the allergic response, but they are not responsible for secreting IgE antibodies.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that IgE is the only antibody class that is primarily produced by plasma cells, whereas other antibody classes, such as IgG and IgM, can be produced by both plasma cells and activated B cells.

**✓ Correct Answer: D. Plasma cells**