The Physical barriers of the human immune system are considered the first line of defense in the protection of the human body as they present a physical barrier that prevents entry of the pathogens.
The skin is one of the physical barriers covering almost the entire body, preventing entry of any bacteria and other pathogens. Sometimes the skin may get punctured or damaged, triggering a consequent chemical barrier in form of enzymes that fight the bacteria.
For those areas of the body not under skin, there are mucous membranes that produce mucous, i.e., a sticky substance capable of trapping any harmful foreign bodies from entering the body. Passageways such as the mouth and the entire digestive system along with the respiratory system are covered with mucous membranes for the same reason. To aid the function of mucous are cilia, hair-like structures on these surfaces that help trap foreign bodies.
Chemical barriers are very important in the body as they protect it from harmful foreign pathogens through chemical reactions that end up destroying the foreign threat.
People are bound to ingest harmful substances mostly found in the food they eat. The foreign bodies that are orally taken in end up in the stomach where hydrochloric acid eventually destroys them. The acid reacts with the foreign bodies, chemically decomposing and destroying them.
Antibodies and enzymes are chemical substances, protein in nature, that are produced whenever harmful substances enter the body. They are effective in fighting infections caused by microbes because they digest them. Body cells produce antibodies and enzymes in the body and release them only when triggered by the presence of pathogens.