Chemical barriers, as related to human immunology, are fatty acids, proteins, bodily secretions, and other substances with natural attributes that help defend the body against disease or infection. Such substances may have antimicrobial properties, low pH, or serve to breakdown or destabilize bacterial cells. Most of these barriers are not designed as primary immune system defenses, but rather have such properties as a secondary function. Few exist solely as a defensive mechanism for the immune system.
The human body has many systems for defending against possible threats or infections, including innate or adapted immunity, active or passive mechanisms, and anatomical, humoral, or cellular barriers. In terms of categorizing chemical barriers, such mechanisms are innate, passive, and belong under the heading of anatomical barriers.
As part of the innate immune system, such barriers are built-in at birth. In other words, the body does not have to adapt the immune system to fight infections using these barriers because they are present before an individual’s first day of life. Categorizing chemical barriers as passive indicates that assisting the immune system is a secondary function. As anatomical barriers, they are present outside of the body's tissues, rather than at the cellular level.
The proteins, acids, secretions, and enzymes that make up chemical barriers are produced to perform specific primary tasks as part of normal or involuntary bodily functions. For example, the body produces sweat as part of its natural cooling system. Perspiration is also a passive barrier for the human immune system because its low pH inhibits bacterial growth.
Other defenses include saliva, tears, and nasal secretions. These substances contain both lysozyme and phospholipase, substances that naturally break down the outer wall and cellular membranes of bacterial cells. Saliva’s primary purpose is to aid digestion, where tears and nasal secretions help flush foreign substances and keep body membranes moist. The fact that these chemicals also have a negative effect on threatening bacteria is more side effect than primary function.
Internal chemical barriers also protect against infection should bacteria or other threats get into internal systems or organs. Proteins in the lungs and gastrointestinal tract, known as defensins, have antimicrobial properties that kill off certain types of bacteria. Other gastrointestinal chemicals compete with infectious cells for nutrients or attach to cell walls, starving out harmful or threatening cells. Like the fatty acids in sweat, gastrointestinal chemicals also have a low pH, which further inhibits the growth of harmful bacteria inside the body.