Answer:
Mucous membranes
Explanation:
The epidermis (external) and mucous membranes (internal) represent the first non-specific barriers that act against invading pathogens and harmful substances. The mucous membranes separate the external environment surrounding the mucous cells from the body's internal environment. These membranes are considered as selectively permeable barriers that act to prohibit the passage of pathogenic microorganisms (e.g., bacteria) and harmful agents (toxic chemical substances).
