**Core Concept**
Peyer's patches are specialized lymphoid follicles located in the ileum of the small intestine, playing a crucial role in mucosal immunity. They are sites where immune cells, such as T and B lymphocytes, interact to recognize and respond to pathogens. Immunoglobulins, or antibodies, are key components of this immune response, with different classes serving distinct functions.

**Why the Correct Answer is Right**
IgA is the primary immunoglobulin present in Peyer's patches, as well as in mucosal secretions, such as saliva, tears, and intestinal fluid. This is because IgA provides a first line of defense against pathogens entering the body through mucosal surfaces. IgA antibodies can neutralize pathogens, prevent their attachment to epithelial cells, and facilitate their removal from the mucosa. The presence of IgA in Peyer's patches enables the immune system to mount an effective response to mucosal infections.

**Why Each Wrong Option is Incorrect**
**Option A:** IgM is the first immunoglobulin produced in response to an infection, but it is not the primary immunoglobulin in Peyer's patches. IgM is primarily found in the blood and lymphoid tissues, where it plays a key role in activating the complement system.

**Option B:** IgG is the most abundant immunoglobulin in the blood, but it is not the primary immunoglobulin in Peyer's patches. IgG provides long-term immunity against infections and is responsible for neutralizing pathogens, but it is not as closely associated with mucosal immunity as IgA.

**Option D:** IgD is primarily found on the surface of mature B cells, where it plays a role in activating these cells upon encountering specific antigens. IgD is not typically associated with mucosal immunity or the presence in Peyer's patches.

**Clinical Pearl / High-Yield Fact**
The presence of IgA in Peyer's patches highlights the importance of mucosal immunity in preventing infections. This is particularly relevant in the context of gastrointestinal infections, where the gut-associated lymphoid tissue (GALT) plays a critical role in recognizing and responding to pathogens.

**✓ Correct Answer: C. IgA**