making killer cells and/or killer antibodies that destroy
the antigens. Normally, a person's adaptive immune sys-
tem cells recognize and do not attack his or her own tis-
sues and chemicals. Let’s look first at innate immunity.
Innate Immunity Includes Two Levels
of Nonspecific Defense
The first line of defense that makes up innate immunity
is a system of physical and chemical barriers that pre-
vent pathogens from entering the body (Figure 12.6).
The physical barriers include the skin and the various
mucous membranes in the nose, upper respiratory tract,
intestines, reproductive system (female), and urinary sys-
tem. With its many layers of closely packed, keratinized
cells, the epidermis (the outer epithelial layer of the skin)
provides a formidable physical barrier to the entrance of
microbes. In addition, continual shedding of the top epi-
dermal cells helps remove microbes at the skin's surface.
Bacteria rarely penetrate an intact and healthy epidermis.
The epithelial layer of mucous membranes secretes a
fluid called mucus that lubricates and moistens the sur-
face of a body cavity. Because mucus is sticky, it traps many
microbes and foreign substances. The mucous membrane
of the nose has mucous-coated hairs that trap and filter
microbes, dust, and pollutants from inhaled air. The mu-
cous membrane of the upper airways contain cilia, micro-
scopic hairlike projections on the surface of the epithelial
cells, which propel inhaled dust and microbes that have
become trapped in mucus toward the throat.
Other fluids such as saliva, sweat, and tears can wash
pathogens away from the skin and mucous membranes.
In addition to these physical barriers, chemicals within
various fluids and secretions can slow the growth of mi-
crobes. Sebaceous (oil) glands of the skin secrete an oily
substance called sebum that forms a protective film over the
surface of the skin. Perspiration helps flush microbes from
the surface of the skin and contains lysozyme, an enzyme
capable of breaking down the cell walls of certain bacteria.
Lysozyme is also found in tears, saliva, nasal secretions, and
tissue fluids. Gastric juice, a mixture of hydrochloric acid,
enzymes, and mucus in the stomach, destroys many bacte-
ria and most bacterial toxins. Vaginal secretions are also
slightly acidic, which discourages bacterial growth.
Digestive system
Acidic gastric juice destroys
most bacteria and toxins in
the stomach.
Vomiting and defecation expel
microbes from the stomach
and intestines.
Vagina
Vaginal secretions flush
microbes out of the vagina.
Vaginal acidity discourages
bacterial growth.
Urethra
Urine flow washes
microbes from the urethra.
Physical barriers:
Chemical barriers:
• Vomiting and defecation.
• Acidic gastric juice.
• Vaginal secretions.
• Vaginal acidity.
• Urine flow.
Immune Reponses Help Protect the Body Against Disease
351
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