Which of the following acts as a mechanical barrier to pathogen entry into the body?

• A. Antibodies
• B. Complement proteins
• C. Macrophages
• D. Intact skin

Answer: (D)

Verified step by step guidance

1. Understand the role of physical and chemical barriers in the immune system. These barriers are the body's first line of defense against pathogens, preventing their entry into the body.
2. Recognize that antibodies are proteins produced by B cells that specifically bind to antigens, helping to neutralize pathogens but do not act as a physical barrier.
3. Know that complement proteins are part of the innate immune system that help destroy pathogens by promoting inflammation and cell lysis, but they do not form a mechanical barrier.
4. Identify macrophages as immune cells that engulf and digest pathogens once they have entered the body, rather than preventing their entry.
5. Conclude that intact skin acts as a mechanical barrier by physically blocking pathogens from entering the body, making it the correct answer.