Innate Immunity

First Line of Immune Defense

The first line of immune defense is made up of physical and chemical barriers such as the skin, sweat, tears, and mucous membranes, which prevent pathogens from reaching more vulnerable areas of the body.

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.

The skin and mucosa are bolstered by many forms of chemical defense that help prevent infection, such as lysozymes. A lysozyme is an enzyme produced by many types of cells in the body and is found in secretions such as saliva, tears, and mucus. Lysozyme breaks down the bonds that make up bacterial cell walls. Similar to skin and mucus, the large intestine is heavily colonized with microbes that feed off of material passing through the alimentary canal. These microbes provide protection by outcompeting pathogens for space and resources.

Physical and Chemical Barriers

Physical and chemical barriers are present all across the body and stop infection by preventing pathogens from reaching vulnerable tissues.