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Specific defense
mechanisms
Nonspecific defense mechanisms
First line of defense:
Anatomic barrier
Second line of defense:
Innate immunity
Third line of defense:
Adaptive immunity
Skin
Mucous membranes
Secretions of skin and mucous
membranes
Phagocytic white blood cells
Antimicrobial proteins
Inflammatory response
Lymphocytes
Antibodies
Figure 1.5 Biological defense mechanisms—nonspecifi c and specifi c defense
mechanisms.
1.2 Layers of the Immune System
h e immune system can be envisioned as a multilayer system, each layer consisting
of diff erent types of defense mechanisms (Kuby et al., 2000; Pathak and Palan,
2005). h e three main layers include the anatomic barrier, the innate immunity,
and the adaptive immunity.
Biological defense mechanisms may be classifi ed into two categories: nonspe-
cifi c and specifi c defense mechanisms (see Figure 1.5). Nonspecifi c defense mecha-
nisms produce the same type of response independent of pathogen that enters the
body. In contrast, specifi c defense mechanisms are based on recognizing particular
pathogens. Each one of these defense mechanisms is explained in the following
sections.
1.2.1 Anatomic Barrier
h e fi rst layer of the biological defense is the anatomic barrier, composed of the
skin and the surface of mucous membranes. Intact skin prevents the eruption of
most pathogens and also inhibits most bacterial growth because of its low pH. In
contrast, many pathogens enter the body by binding or penetrating through the
mucous membranes; thus, the role of these membranes is to provide a number
of nonspecifi c mechanisms that help prevent such invasions. For example, saliva,
tears, and some mucous secretions, which contain antibacterial and antiviral sub-
stances (Kuby et al., 2000; Sompayrac, 2003), wash away potential invaders.
1.2.2 Innate Immunity
Innate immunity (Woods, 1991) refers to all defense mechanisms against foreign
pathogens that individuals are born with. Innate immunity is mainly composed of
the following mechanisms:
Phagocytic barriers . Some specialized cells (like macrophages, neutrophils,
and natural killer cells) are able to ingest foreign substances, including whole
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