Final answer:
Intact skin and mucous membranes are the main nonspecific defense mechanisms that provide a physical barrier to prevent microbial invasion in the body, with the skin being difficult to penetrate and mucous membranes trapping and removing pathogens.
Step-by-step explanation:
The main nonspecific defense mechanism against invasion by microbes is intact skin and mucous membranes. These physical barriers, including the skin and mucous membranes, act instantaneously to prevent pathogenic invasion into the body tissues. The skin, being the body's largest organ provides an outer layer that is tough and waterproof, making it very difficult for pathogens to penetrate. The mucous membranes, on the other hand, secrete mucus which traps pathogens and other foreign particles, with cilia in the respiratory tract helping to push this mucus out of the body. Moreover, the skin creates an inhospitable environment for many pathogens by drying out and maintaining an acidic pH, and the competitive presence of beneficial microorganisms on the skin further helps in warding off potential infections.