ers called antigens. Lymphocytes, natural killer
(NK) cells, macrophages, and the complement fac-
tors—a collection of substances that, when acti-
vated (the COMPLEMENT CASCADE), form potent
chemical structures—key onto these antigens like
lasers onto targets. Cells bearing self antigens con-
tinue unimpeded about their business in the body.
Those bearing nonself antigens are tagged for
destruction by another set of molecular markers,
antibodies, that specialized B-cell lymphocytes
called PLASMAcells produce. Each individual anti-
gen generates a different ANTIBODY; millions of
antibodies circulate in the blood and lymph.
Filling out the immune system’s defense are
specialized clusters of lymphoid tissue that line
each point of access into the inner body: SKIN,
NOSE, airways, gastrointestinal system, and even
the blood vessels. These clusters—known collec-
tively as the mucosa-associated lymphoid sys-
tem—are like guard posts protecting the body’s
vulnerabilities. MALT contains abundant popula-
tions of lymphocytes, mast cells, and macrophages
that detect and intercept millions of microbes,
viruses, toxins, and irritants before they can
breach the inner body.
Health and Disorders of the Immune System
In health the immune system is an amazing net-
work of cells and molecules that patrol every
pathway of the body. Most of the time, the
immune system goes about protecting and ridding
the body from invading pathogens without draw-
ing any notice to its activities. Only when the
immune system is too efficient—causing hyper-
sensitivity reactions or autoimmune disorders—or
ineffective—allowing infection or cancer—does its
existence become unpleasantly apparent.
Inflammation: the front line The immune sys-
tem’s first response primary weapon is INFLAMMA-
TION. Inflammation floods the affected tissues with
infection-fighting molecules. Plasma, the liquid
component of blood, carries these molecules and
delivers them to the site of the infection. The
familiar swelling of inflammation is the body’s
“caution: IMMUNE RESPONSEat work” sign. Inflam-
mation causes PAIN, which encourages if not forces
limited activity of the affected area. This allows
plasma to thoroughly saturate the area, speeding
healing and recovery. Inflammation may also
cause FEVER, yet another inducement to take it
easy. As unpleasant as these symptoms are, they
serve notice that the immune system is on task.
Autoimmune disorders: attacking self cells
Inflammation sometimes gets out of hand. The
immune response may not recognize that its task
is over. Inflammation may become so severe that
it impedes the flow of blood, threatening the well-
being of the body in other ways. ANTIBIOTIC MED-
ICATIONSmay be necessary to bring in another
flank of attack against the PATHOGEN. Or a malfunc-
tion of the immune system may erroneously mark
self cells as invaders, directing the immune
response to attack structures of the body that con-
tain the cells. These are autoimmune disorders—
such as RHEUMATOID ARTHRITIS, SYSTEMIC LUPUS
ERYTHEMATOSUS(SLE), and type 1 DIABETES.
Allergies: mistaken identity About 50 million
Americans have allergies, mostly to pollens,
molds, foods, animal dander, and medications.
More than 18 million adults and 7 million chil-
dren in the United States have seasonal allergies
that cause ALLERGIC RHINITIS, ALLERGIC ASTHMA, and
ALLERGIC CONJUNCTIVITIS—a triad of hypersensitivity
reactions known collectively as hay fever. Another
9 million Americans, two thirds of them children,
have FOOD ALLERGIES.
With allergies, the immune system turns not
against the body itself but against harmless sub-
stances the body encounters, misidentifying them
as dangerous intruders. Plasma cells then generate
antibodies that perceive the molecular markers of
these substances—allergens—as harmful and
launch an immune response upon detecting their
presence. Hypersensitivity reactions cause symp-
toms that range from annoying to life-threatening.
It often is not possible to escape the reach of an
ALLERGEN; treatment becomes the only recourse for
relieving symptoms.
Immunodeficiency: AWOL Sometimes the
immune system fails to function properly because
key components are deficient or missing. Genetic
errors may result in an absence of T-cell lympho-
cytes, B-cell lymphocytes, complement factors,
immunoglobulins, or other substances necessary
to integrate the immune response. Such deficits
increase vulnerability to infection and, when
severe, threaten life. The most significant acquired
immunodeficiency is HIV/AIDS, which results from236 The Immune System and Allergies