What structures provide physical or chemical barriers against microbial invasion?

Physical barriers

* Skin: The skin is the largest organ of the body and acts as a physical barrier to microbial invasion. It is composed of multiple layers of cells that produce antimicrobial substances and secrete fatty acids that can kill or inhibit the growth of microorganisms.

* Mucous membranes: Mucous membranes are thin, moist tissues that line the respiratory, digestive, and urogenital tracts. They produce mucus, which is a sticky substance that traps microorganisms and prevents them from entering the body.

* Saliva: Saliva contains antimicrobial substances such as lysozyme and lactoferrin that can kill or inhibit the growth of microorganisms.

* Stomach acid: The stomach produces hydrochloric acid, which has a pH of about 1.5. This acidic environment can kill or inhibit the growth of most microorganisms.

* Intestinal flora: The intestines contain a large population of beneficial bacteria that help to protect the body against harmful microorganisms. These bacteria compete with harmful microorganisms for nutrients and produce antimicrobial substances.

Chemical barriers

* Antimicrobial peptides: Antimicrobial peptides are small proteins that are produced by the body to kill or inhibit the growth of microorganisms.

* Cytokines: Cytokines are proteins that are produced by the immune system to regulate the immune response. They can also activate antimicrobial peptides and other immune cells.

* Complement system: The complement system is a group of proteins that work together to kill microorganisms. It is activated by the presence of foreign molecules, such as those found on the surface of bacteria.