In the first line of defense against infection, physical and anatomic barriers try to prevent the entry of pathogens. The skin and the surface of mucous membranes effectively prevent the entry.
The skin consists of two distinct layers. A thinner outer layer is called the epidermis and a thicker inner layer is called the dermis.The epidermis contains many layers of tightly packed epithelial cells. The outermost epidermal layer consists of dead cells and is filled with a hydrophobic protein called keratin. The dermis, which is composed of connective tissue contains blood vessels, hair follicles, sebaceous glands, and sweat glands. The sebaceous glands are associated with the hair follicles and produce an oily secretion called sebum. Sebum consists of lactic acid and fatty acids. These acids maintain the pH of the skin between 3 and 5. This low pH inhibits the growth of most microorgan-isms. Intact skin prevents the entry of pathogens and also its low pH inhibits most bacterial growth. Breaks in the skin, even small ones, result in the entry of pathogens to cause infections. The skin is also penetrated by biting insects (mosquitoes, mites, ticks, fleas, flies); if these insects harbor pathogenic organisms, they can introduce the patho-gens into the body as they feed.
The conjunctiva and the alimentary, respiratory, urogenital tracts are lined by mucous membranes. These membranes consist of an outer epithelial layer and an underlying layer of connective tissue. A number of nonspecific defense mechanisms exist to prevent the entry of patho-gens. For example, saliva, tears, and mucous secretions act to wash away potential invaders. They contain antibacterial and antiviral sub-stances. The viscous fluid called mucus, which is secreted by epithelial cells entraps foreign microorganisms. In the respiratory and gastrointes-tinal tract, the mucus membrane is covered by cilia. The synchronous movement of cilia propels mucus-entrapped microorganisms towards the exterior from these tissues. In addition, nonpathogenic organisms tend to colonize epithelial cells of mucosal surfaces. These normal flora generally compete with pathogens for attachment sites on the epithelial cell surface and for necessary nutrients. Mucus also contains the en-zyme lysozyme which lyses the bacterial cell. Mucoproteins present in the mucus also inhibit the haemagglutinins of influenza virus
Many pathogens enter the body by binding to and penetrating mucous membranes, overcoming the protective effects.