Protection From Infection: Non-specific Defenses
The body is in constant contact with pathogens, microorganisms that can cause disease. These are mainly bacteria and viruses, although some pathogens are protozoa, fungi, or even worms. In any event, the body possesses two major lines of defense to protect itself, the non-specific and specific defenses. The non-specific defenses protect the body from pathogens in general, while the specific defenses, often called the immune response, target and destroy one particular type of pathogen that may be infecting the body at any given time. We will focus on the specific defense shortly.
The non-specific defenses include any physical or chemical barriers able to prevent the colonization of the body by pathogens (infection). The skin and mucous membranes provide a continuous surface in direct contact with the environment that should physically prevent the entrance of pathogens. Intact skin is more reliable than the mucous membranes, however, which often must function in the exchange of materials with the environment and tend to be more permeable. Because of this vulnerability, the mucous membranes secrete mucus, a sticky substance designed to trap pathogens before they can breach the membrane. Even so, some microorganisms are able to enter the body through intact mucous membranes, especially in the respiratory tract. Additionally, skin may be punctured or cut so that it is no longer intact, making an inviting target of entry for potential pathogens.
Chemical non-specific defenses include the following:
- enzymes, such as lysozyme, can kill bacterial cells and are present in tears and saliva.
- perspiration, secreted from sweat glands, creates osmotically unfavorable conditions for bacterial growth due its high salt concentration.
- acidic conditions in the stomach kill most of the bacteria ingested in food before they can proliferate.
- interferons are small proteins effective only against viruses that are released by virally infected cells to signal their neighbors to prepare for invasion.
- If a pathogen manages to breach these first lines of defense and enters the body (usually at the skin or mucous membranes), it must deal with the second line of non-specific defenses: phagocytosis and inflammation. As noted previously, phagocytes are neutrophils and macrophages that can engulf and digest foreign invaders. If pathogens enter through damaged tissue (as with a skin wound), phagocytosis is coupled to a more dramatic inflammatory response. Damaged cells release chemicals that have multiple effects:
- blood supply to the area is increased and clots often form, isolating the damaged area.
- phagocytic cells are recruited and enter the region, digesting the invaders they find.
- local temperature may rise, inhibiting the growth of pathogenic bacteria.
- The inflammatory response, while usually painful, is certainly beneficial: it is designed to disable intruders and promote tissue repair before the pathogens can enter the bloodstream. If this defense fails, a systemic infection may result, and the final line of defense is the immune response.