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15.1 DEFENSE MECHANISMS
Nonspecific immune protection is provided by such mech-
anisms as phagocytosis, fever, and the release of inter-
ferons. Specific immunity, which involves the functions of
lymphocytes, is directed at specific molecules or parts of
molecules known as antigens.
The immune system includes all of the structures and processes
that provide a defense against potential pathogens (disease-causing
agents). These defenses can be grouped in two categories: innate
(or nonspecific ) immunity and adaptive (or specific ) immunity.
Although these two categories refer to different defense mecha-
nisms, there are areas in which they overlap.
Innate, or nonspecific, defense mechanisms are inherited as
part of the structure of each organism. Epithelial membranes,
including the epidermis of the skin and the mucous membranes
of the gastrointestinal, reproductive, respiratory, and genitouri-
nary tracts, cover all body surfaces. In addition to being a physi-
cal barrier of high surface area (2 m^2 for the skin and 200 m^2 for
the intestine) to pathogenic microbes, these epithelial membranes
secrete a variety of antimicrobial peptides. The strong acidity
of gastric juice (pH 1–2) also helps kill many microorganisms
before they can invade the body. These external defenses are
backed by internal defenses, such as phagocytosis, which func-
tion in both a specific and nonspecific manner ( table 15.1 ).
Each individual can acquire the ability to defend against
specific pathogens by prior exposure to those pathogens. This
adaptive, or specific, immune response is a function of lym-
phocytes. Internal specific and nonspecific defense mecha-
nisms function together to combat infection, with lymphocytes
interacting in a coordinated effort with phagocytic cells.Eleven-year-old Timmy steeled himself for his Tdap
booster shot. His mother, a nurse, patiently explained
that it was a vaccine containing inactivated toxins from
bacteria, and that it would protect him from tetanus,
diphtheria, and pertussis (whooping cough). He didn’t
know what that meant, but was happy the ordeal was
over. However, a week later he was unhappy again when
he cut his hand on an old can in the road when he fell
off his bike. His mother cleansed the wound, dabbed it
with a bactericidal salve, and bandaged it, saying, “We
don’t want the infection to spread.” The wound hurt and
got red, swollen, and oozed some pus, but it eventually
healed. Timmy was again happy, except for his sea-
sonal cough, for which his mother gave him a children’s
antihistamine.
Some of the new terms and concepts you will
encounter include:- Local inflammation; innate and adaptive immunity
- Active immunity and the clonal selection theory
- Immunoglobulins and immediate hypersensitivity
Clinical Investigation
Structure Mechanisms
External Skin Physical barrier to penetration by pathogens; secretions contain lysozyme (enzyme that destroys
bacteria)
Digestive tract High acidity of stomach; protection by normal bacterial population of colon
Respiratory tract Secretion of mucus; movement of mucus by cilia; alveolar macrophages
Genitourinary tract Acidity of urine; vaginal lactic acid
Internal Phagocytic cells Ingest and destroy bacteria, cellular debris, denatured proteins, and toxins
Interferons Inhibit replication of viruses
Complement proteins Promote destruction of bacteria; enhance inflammatory response
Endogenous pyrogen Secreted by leukocytes and other cells; produces fever
Natural killer (NK) cells Destroy cells infected with viruses, tumor cells, and mismatched transplanted tissue cells
Mast cells Release histamine and other mediators of inflammation, and cytokines that promote adaptive immunityTable 15.1 | Structures and Defense Mechanisms of Nonspecific (Innate) Immunity
LEARNING OUTCOMESAfter studying this section, you should be able to:- Describe the different elements of the innate immune
system. - Describe the nature of antigens, lymphocytes, and
lymphoid organs. - Explain the events that occur in a local inflammation.