ciliated epithelium of the upper respiratory tract is an
especially effective barrier. Dust and pathogens are
trapped on the mucus, the cilia sweep the mucus to the
pharynx, and it is swallowed. The hydrochloric acid of
the gastric juice destroys most pathogens that enter
the stomach, either in mucus or with food and drink.
Lysozyme, an enzyme found in saliva and tears,
inhibits the growth of bacteria in the oral cavity and
on the warm, wet surface of the eye. The subcuta-
neous tissue contains many white blood cells (WBCs),
as does the areolar connective tissue below the epithe-
lium of mucous membranes.
Defensive Cells
Recall from Chapter 11 that many of our defensive
cells are white blood cells. Macrophages, both fixed
and wandering, have receptors for the pathogens
humans are likely to encounter (this probably reflects
millions of years of coexistence) and are very efficient
phagocytes. Other cells capable of phagocytosis of
pathogens or other foreign antigens are the neutro-
phils and, to a lesser extent, the eosinophils. Phago-
cytic cells use intracellular enzymes and chemicals
such as hydrogen peroxide (H 2 O 2 ) to destroy ingested
pathogens.
The Langerhans cells of the skin, and other den-
dritic cells throughout the body, also phagocytize for-
eign material, not merely to destroy it, but to take it to
a lymph node where the lymphocytes of adaptive
immune mechanisms are then activated. The macro-
phages are also involved in activating these lympho-
cytes. This is a very important link between the two
components of immunity.
Natural killer cells(NK cells) circulate in the
blood but are also found in the red bone marrow,
spleen, and lymph nodes. They are a small portion
(about 10%) of the total lymphocytes, but are able to
destroy many kinds of pathogens and tumor cells. NK
cells make direct contact with foreign cells, and kill
them by rupturing their cell membranes (with chemi-
cals called perforins) or by inflicting some other kind
of chemical damage.
Basophils and mast cells (a type of connective tissue
cell) are also defensive cells that are found throughout
areolar connective tissue. They produce histamine and
leukotrienes. Histamine causes vasodilation and makes
capillaries more permeable; these are aspects of
inflammation. Leukotrienes also increase capillary
permeability and attract phagocytic cells to the area.
Chemical Defenses
Chemicals that help the body resist infection include
the interferons, complement, and the chemicals
involved in inflammation. The interferons(alpha-,
beta-, and gamma-interferons) are proteins produced
by cells infected with viruses and by T cells. Viruses
must be inside a living cell to reproduce, and although
interferon cannot prevent the entry of viruses into
cells, it does block their reproduction. When viral
reproduction is blocked, the viruses cannot infect new
cells and cause disease. Interferon is probably a factor
in the self-limiting nature of many viral diseases (and
is used in the treatment of some diseases, such as hep-
atitis C).
Complementis a group of more than 20 plasma
proteins that circulate in the blood until activated.
They are involved in the lysis of cellular antigens and
the labeling of noncellular antigens. Some stimulate
the release of histamine in inflammation; others
attract WBCs to the site.
Inflammationis a general response to damage of
any kind: microbial, chemical, or physical. Basophils
and mast cells release histamine and leukotrienes,
which affect blood vessels as previously described.
Vasodilation increases blood flow to the damaged area,
and capillaries become more permeable; tissue fluid
and WBCs collect at the site. The purpose of inflam-
mation is to try to contain the damage, keep it from
spreading, eliminate the cause, and permit repair of the
tissue to begin. From even this brief description you
can see why the four signs of inflammation are redness,
heat, swelling, and pain: redness from greater blood
flow, heat from the blood and greater metabolic activ-
ity, swelling from the accumulation of tissue fluid, and
pain from the damage itself and perhaps the swelling.
As mentioned in Chapter 10, inflammation is a pos-
itive feedback mechanism that may become a vicious
cycle of damage and more damage. The hormone cor-
tisol is one brake that prevents this, and at least one of
the complement proteins has this function as well.
There are probably other chemical signals (in general
called cytokinesand chemokines) that help limit
inflammation to an extent that is useful.
In summary, innate immunity is nonspecific, is
always the same, does not create memory, and does
not become more efficient upon repeated exposures.
Some cells of innate immune mechanisms also activate
the adaptive immune mechanisms. The aspects of
innate immunity are shown in Fig. 14–6.328 The Lymphatic System and Immunity