Human Immunology: Nonspecific Immune System Defenses

Although hand washing and use of hand sanitizer reduces your chances of catching an infection, it is impossible to avoid germs. Pathogenic microbes (those that can cause disease) are all around us, all the time. It is the body's own immune system that does the most to protect humans from infectious disease. The first line of immune system defense consists of physical and chemical components of the skin and mucous membranes that provide general protection by working to prevent microbes from entering the body.

Physical Components of the Skin's Defense

Skin is a physical barrier comprised of two main layers:
* epidermis
* dermis

The Epidermis: Few pathogens can penetrate this outer part of the skin, which is made up of multiple layers of tightly packed cells. In addition to this structural barrier, the natural shedding of dead skin cells removes many attached microorganisms. Within the epidermis are specialized cells, called epidermal dendritic cells, these cells actively patrol the skin to phagocytize (engulf and digest) pathogens that have breached the skins outer boundary.

The Dermis: The dermis is situated beneath the epidermis and contains protein fibers called collagen. Collagen is a tough, fibrous protein which gives skin the strength and pliability to resist abrasions that could introduce microorganisms.

Chemical Components of the Skin's Defense

Chemical secretions of the skin, such as sweat and oil, also function to thwart a microbial invasion.

Perspiration: Secreted by the skin's sweat glands, perspiration contains salt and enzymes. Few microbes can live in a highly saline environment, like that of the skin's surface. Sweat is also supplemented with lysozyme, a type of enzyme that can destroy the cell walls of bacteria.

Sebum: Secreted by skins sebaceous (oil) glands, sebum helps keep skin pliable and less likely to break or tear. Sebum also lowers the pH of the skin to a more acidic level that inhibits the growth of many types of bacteria.

The Role of Mucous Membranes

Mucus-secreting membranes line all body cavities open to the outside environment. Unlike epidermal cells in the skins surface, the epithelial cells of mucous membranes are living.The mucus secreted by these specialized cell helps trap invading pathogens much the way fly-paper traps flying insects. Like epidermal cells, epithelial cells are tightly packed together, preventing entry of pathogens into the body. Epithelial cells are also continually shedding; the discarded cells carrying away any attached microorganisms.

Sources

Bauman, R. (2005) Microbiology. Pearson Benjamin Cummings.
Park Talaro, K. (2008) Foundations in Microbiology. McGraw Hill.