Barriers to infection

The body has many physical, chemical and biochemical barriers that make it much more difficult for pathogens to gain entry into the body.

Physical and chemical defence mechanisms.

The physical barriers to infection are as follows:
• Skin and mucosa. Intact skin and mucosa provide a physical barrier to prevent entry of organisms.
• Cilia. The respiratory tract is lined with little hair-like structures that beat in such a way as to propel particles towards the throat, where they can be expelled by coughing or swallowing and excretion.
• Mucus. Mucus is secreted by epithelial cells of the gut, respiratory tract and genito-urinary (GU) tract. It has the unusual properties of being sticky and slimy at the same time and is able to entrap microorganisms so they can be expelled. In the respiratory tract, cilia and mucus combine to provide an effective way of trapping and eliminating microbes.

The chemical and biochemical defences are as follows:
• Acids. Hydrochloric acid secreted by the stomach is lethal to many (though not all) bacteria. Commensal bacteria in the vagina produce lactic and proprionic acid resulting in a low pH, which is inhibitory to the division of many bacteria.
• Fatty acids. Sebaceous glands in the skin produce fatty acids that have antimicrobial properties.
• Lysozyme. This is present in sweat, tears and many other secretions. It breaks down peptidoglycans in bacterial cell walls, thus damaging and killing the bacteria.
• Defensins. Defensins are antimicrobial peptides that are found in the secretions of mucosa and skin.
• Cathelicidins. These antibacterial peptides were originally discovered as insect defence peptides. Other members of the cathelicidin family have been found in mucosal secretions.
• Collectins. Collectins are proteins that can bind sugars on microbial surfaces and promote the elimination of microbes. Proteins that bind sugars are known as lectins; because collectins bind sugars in a calciumdependent manner, they are known as C-type lectins. The A and D lung surfactants are collectins that provide protection at the lung surface; other collectins, such as mannose-binding protein, are found in serum.

The physical and chemical barriers are very effective at preventing pathogens from entering the body and they exclude more than 99.9% of the infectious organisms we are exposed to. However, organisms do infect the body. This can occur in a number of ways.