**Core Concept**
The mucosal immune system is a unique subset of the immune system that is responsible for protecting the body's mucosal surfaces, such as the respiratory, gastrointestinal, and genitourinary tracts, from pathogens and other foreign substances.

**Why the Correct Answer is Right**
The mucosal immune system is characterized by the presence of a large number of immune cells, including dendritic cells, T cells, and B cells, that are specifically adapted to function in this environment. These cells produce a variety of cytokines and chemokines that help to recruit other immune cells to the site of infection and to activate them to produce a protective response. One key feature of the mucosal immune system is the presence of a specialized type of immune cell called a mucosal-associated lymphoid tissue (MALT) cell, which is capable of producing large amounts of IgA, a type of antibody that is particularly effective at neutralizing pathogens at mucosal surfaces.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not describe a characteristic of the mucosal immune system. While the lymph nodes are an important part of the immune system, they are not specifically adapted to function in the mucosal environment.

**Option B:** This option is incorrect because it describes a characteristic of the systemic immune system, not the mucosal immune system. The systemic immune system is responsible for protecting the body from pathogens and other foreign substances that enter through the bloodstream, but it is not as specialized for function at mucosal surfaces.

**Option C:** This option is incorrect because it does not describe a characteristic of the mucosal immune system. While the spleen is an important part of the immune system, it is not specifically adapted to function in the mucosal environment.

**Option D:** This option is incorrect because it does not describe a characteristic of the mucosal immune system. While the lymphoid follicles are an important part of the immune system, they are not specifically adapted to function in the mucosal environment.

**Clinical Pearl / High-Yield Fact**
One key feature of the mucosal immune system is its ability to produce large amounts of IgA, which is particularly effective at neutralizing pathogens at mucosal surfaces. This is why vaccination with live, attenuated pathogens is often effective at inducing mucosal immunity.

**Correct Answer:** C.