**Core Concept**
The gastrointestinal tract produces immunoglobulins, particularly those that provide protection against pathogens entering the body through the mucosal surface. The intestinal immune system is characterized by the production of specific immunoglobulins that neutralize or remove pathogens from the gut lumen.

**Why the Correct Answer is Right**
The correct immunoglobulin secreted by the intestine is IgA. It is the predominant immunoglobulin found in mucosal secretions, such as saliva, tears, and intestinal mucus. IgA prevents pathogens from adhering to the intestinal epithelial cells, thereby reducing the risk of infection. The secretory component of IgA, which is produced by the intestinal epithelial cells, plays a crucial role in transporting IgA across the epithelial barrier into the gut lumen.

**Why Each Wrong Option is Incorrect**
* **Option A:** IgG is the most abundant immunoglobulin in the serum and plays a key role in the secondary immune response, but it is not the major immunoglobulin secreted by the intestine.
* **Option B:** IgM is the first immunoglobulin produced in response to an infection and is primarily involved in the primary immune response, but it is not the major immunoglobulin secreted by the intestine.
* **Option C:** IgE is involved in the allergic response and is not secreted by the intestine in significant amounts.

**Clinical Pearl / High-Yield Fact**
The secretory component of IgA is crucial for its transport across the intestinal epithelial barrier, and defects in this component can lead to impaired IgA secretion and increased susceptibility to gastrointestinal infections.

**Correct Answer: D. IgA**