Skin

The skin provides the most difficult barrier for microbes to penetrate; it is composed of two main layers—the dermis and the epidermis (see figure 22.1). The dermis contains tightly woven

Stratified epithelium; skin (the outer cell layers are embedded with keratin), lining of the mouth

Figure 15.2 Epithelial Barriers Cells of these barriers are tightly packed together and rest on a layer of thin fibrous material, the basement membrane, helping prevent entry of materials through the barrier. Note the cilia on some epithelial cells propel material to an area where it can be eliminated.

Stratified epithelium; skin (the outer cell layers are embedded with keratin), lining of the mouth

Basement membrane

- Connective tissue

Nucleus

Columnar cell

Mucus-producing cell

Columnar epithelium; passages of respiratory system, various tubes of the reproductive systems

Columnar cell

Mucus-producing cell

Columnar epithelium; passages of respiratory system, various tubes of the reproductive systems

Figure 15.2 Epithelial Barriers Cells of these barriers are tightly packed together and rest on a layer of thin fibrous material, the basement membrane, helping prevent entry of materials through the barrier. Note the cilia on some epithelial cells propel material to an area where it can be eliminated.

Nucleus

Antimicrobial factors in saliva (lysozyme, peroxidase, lactoferrin)

Lysozyme in tears and other secretions and in phagocytes

Removal of inhaled particles

Skin-physical barrier, fatty acids, sweat, normal flora

Rapid pH change from stomach to upper intestine pH and normal flora of vagina

Antimicrobial factors in saliva (lysozyme, peroxidase, lactoferrin)

Lysozyme in tears and other secretions and in phagocytes

Removal of inhaled particles

Skin-physical barrier, fatty acids, sweat, normal flora

Rapid pH change from stomach to upper intestine pH and normal flora of vagina

Acid in stomach (low pH)

Flushing of urinary tract

Figure 15.3 First-Line Defense Mechanisms in Humans Physical barriers, such as skin and mucous membranes, antimicrobial secretions, and normal flora work together to prevent entry of microorganisms into the host's tissues.

Acid in stomach (low pH)

Flushing of urinary tract

Figure 15.3 First-Line Defense Mechanisms in Humans Physical barriers, such as skin and mucous membranes, antimicrobial secretions, and normal flora work together to prevent entry of microorganisms into the host's tissues.

fibrous connective tissue, making it extremely tough and durable; the dermis of cows is used to make leather. The epidermis is composed of many layers of epithelial cells, which become more progressively flattened towards the exterior. The outermost sheets are made up of dead cells that have been embedded with a water-repelling protein called keratin, resulting in the skin being an arid environment. The cells continually slough off, taking with them any microbes that might be adhering. ■ anatomy and physiology of the skin, p. 533

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