Which of the following act as barriers to reduce the entry of pathogens?

The human body has several mechanisms that act as barriers to reduce the entry of pathogens. These include:

1. Skin: The skin acts as a physical barrier, preventing pathogens from entering the body. It is also slightly acidic which discourages the growth of certain bacteria.

2. Mucous Membranes: These line various parts of the body such as the nose, mouth, and lungs. They produce mucus which can trap pathogens. Some mucous membranes also have cilia, tiny hair-like structures that can move in coordinated waves to push the mucus and trapped pathogens out of the body.

3. Stomach Acid: The stomach produces acid which can kill many pathogens that are ingested with food.

4. Saliva and Tears: Both of these fluids contain enzymes that can break down the cell walls of many bacteria, killing them.

5. Friendly Bacteria: The body also has colonies of 'friendly' bacteria, particularly in the gut, that can out-compete pathogenic bacteria for resources, preventing them from multiplying.

6. Immune System: If pathogens do manage to enter the body, the immune system can recognize and destroy them. This includes both the innate immune system, which can respond to a wide range of pathogens, and the adaptive immune system, which can 'learn' to recognize specific pathogens and provide immunity against them.