Final answer:
The skin is the part of the immune system that acts as a physical barrier, protecting against the entry of pathogens and forming an essential component of the innate immune system.
Step-by-step explanation:
The part of the immune system that is a physical barrier is the skin. The skin is an essential component of the innate immune system, which includes various barrier defenses always working to protect the body against pathogens. The skin functions as a continuous, impassable barrier that prevents potentially infectious pathogens from entering the body. It is tough and made of cells that are constantly being shed, helping to remove bacteria and other infectious agents.
Other physical barrier defenses include hair in the nasal passages that trap particles, mucous membranes, and fluids such as tears and urine. These not only block pathogens mechanically but also incorporate chemical elements that help to incapacitate and remove pathogens from the body.