**Core Concept**
The concept of the greatest barrier to infection refers to the body's natural defense mechanisms that prevent pathogens from entering and causing disease. The primary barrier to infection is the skin and mucous membranes, which provide a physical and chemical barrier against pathogens.

**Why the Correct Answer is Right**
The correct answer is the skin. The skin acts as the first line of defense against infections, preventing pathogens from entering the body. It achieves this through several mechanisms, including the presence of sweat glands, sebaceous glands, and hair follicles, which produce antimicrobial peptides and lipids that inhibit the growth of microorganisms. Additionally, the skin's acidic pH and the presence of lysozyme also contribute to its antimicrobial properties.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the mucous membranes, while important in preventing infection, are not the greatest barrier to infection. They are more susceptible to infection than the skin and are often targeted by pathogens.
**Option B:** This option is incorrect because the immune system, while crucial in fighting infections, is not the primary barrier to infection. It is a secondary defense mechanism that is activated once pathogens have breached the skin or mucous membranes.
**Option C:** This option is incorrect because the respiratory and gastrointestinal tracts are more susceptible to infection than the skin and mucous membranes. They are often targeted by pathogens and are associated with a higher risk of infection.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the skin is the body's first line of defense against infections, and any breach in the skin's integrity can lead to the entry of pathogens and the development of disease.

**Correct Answer:** C.