Final answer:
Protective barriers against pathogens are generally categorized into physical barriers like the skin and chemical barriers such as bodily secretions with antimicrobial properties.
Step-by-step explanation:
Protective barriers can be broadly classified into two major categories: physical barriers and chemical barriers. These barriers serve as the body's first line of defense against pathogens.
Physical Barriers
Mechanical barriers, such as skin, act as a form of physical block, preventing pathogens from entering the body. The skin is not only a tough barrier difficult for pathogens to penetrate, but it also sheds cells that can carry away potential invaders.
Chemical Barriers
Chemical barriers include bodily secretions that can lower pH or contain substances toxic to microbes, inhibiting their growth or destroying them outright.
Biological Barriers
Although not one of the main two categories, it is worth mentioning that biological barriers also play a role. These living organisms, such as the harmless bacteria on our skin and in our gastrointestinal tract, occupy spaces and utilize resources, making it less feasible for harmful bacteria to thrive and grow.