The first line of defense protecting the human body is the physical and chemical barriers that stop pathogens from reaching more vulnerable areas. This is the primary function of the largest organ in the human body, the skin. Human skin is made up of several layers of different types of cells topped by a layer of dead skin cells. The skin provides protection through desquamation, a process of shedding the outermost layer of skin to protect the body from microbes bound to the skin. This outer layer of dead skin cells is dry, salty, and water-resistant, which makes it very difficult for most potential pathogens to break through to the bloodstream. In addition, the skin is colonized by a large and diverse community of commensal microbes that feed on dead cells and cellular detritus. These bacteria take up physical space and resources and thus protect the body from pathogen colonization. Pathogens must invade the skin through breaks caused by cuts, burns, or insect bites. The colonization of the skin by nonpathogenic microbes reduces the probability that a break in the skin will allow access to a pathogen. The skin tissue itself can also be infected by microbes that cause boils, cold sores, and athlete's foot.
Areas of the body that are non-sterile or indirectly exposed to the outside environment, such as the alimentary canal of the digestive system (ingestion of food), the lungs, and the nasal passages (breathing of air), are covered in a different type of skin called a mucous membrane, or the mucosa. The mucosa needs to be kept moist at all times in order to absorb nutrient molecules or oxygen and thus lacks a layer of dead skin cells to protect it. The mucous membranes make up for this with the production of mucus, a slimy material made from long chains of carbohydrates that is loosely associated with the mucosa and is sloughed off over time. Like the skin, mucus is densely colonized by bacteria that consume it for energy, helping to protect their host.
The mucosa is also lined with mucosa-associated lymphoid tissue (MALT), clusters of immune cells located in the mucous areas of the body (lungs, eyes, nasal passages) that protect the body from infection by pathogens. These cells monitor chemical signals that pass by them to watch out for potential pathogens and mount an immune response if they are activated. Respiratory tract mucous membrane is lined with ciliated epithelial cells that beat constantly and push any trapped particles upward. This mucociliary escalator helps to keep the lower respiratory tract mostly free of pathogens.
The MALT in the alimentary canal, or digestive tract, is called gut-associated lymphoid tissue (GALT) and provides defense in this organ system that is particularly vulnerable invasion. One aspect of GALT is a Peyer's patch, a small cluster of immune cells located along the wall of the small intestine that prevents infection by pathogens passing through the gastrointestinal tract. Stomach acids also provide a general defense by preventing the growth of pathogens since the low pH is below the growth optimum for most pathogens and denatures proteins.