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 chemical barriers are not designed as primary immune system defenses, but rather have such properties as a secondary function. Few such barriers exist solely as a defensive mechanism for the immune system.
The human body has numerous systems for defending against possible threats or infections. Such systems include 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, chemical barriers are built-in at birth. In other words, the body does not have to adapt the immune system to fight infections using chemical barriers because such barriers 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. The classification of chemical barriers under the heading of anatomical barriers indicates that such defenses are present outside of body tissues, rather than at the cellular level.
Although part of the innate immunological system, chemical barriers are not designed solely to protect against infection, but rather exhibit passive immunity characteristics as a secondary function. Proteins, acids, secretions, and enzymes that encompass 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 chemical barrier for the human immune system because its low pH inhibits bacterial growth.
Other chemical barriers 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 chemical barriers also have a negative effect on threatening bacteria is more side effect than primary function, again illustrating the passive nature of chemical barriers.
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 characteristics and properties that kill off certain types of bacteria. Other gastrointestinal chemicals compete with infectious cells for nutrients or attach to cell walls, thus starving out harmful or threatening cells. Like the fatty acids in sweat, gastrointestinal chemical barriers also have a low pH, which further inhibits the growth of harmful bacteria inside the body.
