Reading Comprehension Success in 20 Minutes a Day, 3rd Edition

a. the continued existence of national parks is
threatened by budget cuts.
b. the best way to preserve the environment on
Mt. Desert Island is to limit the number of
visitors.
c. national parks allow large numbers of people
to visit and learn about interesting wilderness
areas.
d. Mt. Desert Island is the most interesting
tourist attraction in Maine.
41. According to the selection, the coast of Maine is
a. 2,500 miles long.
b. 3,500 miles long.
c. 225 miles long.
d. 235 miles long.
42. What is the meaning of the underlined phrase
flora and fauna in paragraph 4 of this passage?
a. insects and plants 
b. plants and animals
c. deer and coyote 
d. birds and beaches 
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P O S T T E S T
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1 6 4

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The immune system is equal in complexity to the combined intricacies of the brain and nervous system. The suc-
cess of the immune system in defending the body relies on a dynamic regulatory communications network con-
sisting of millions and millions of cells. Organized into sets and subsets, these cells pass information back and forth
like clouds of bees swarming around a hive. The result is a sensitive system of checks and balances that produces
an immune response that is prompt, appropriate, effective, and self-limiting.
At the heart of the immune system is the ability to distinguish between self and nonself. When immune
defenders encounter cells or organisms carrying foreign or nonself molecules, the immune troops move quickly
to eliminate the intruders. Virtually every body cell carries distinctive molecules that identify it as self. The
body’s immune defenses do not normally attack tissues that carry a self-marker. Rather, immune cells and other
body cells coexist peaceably in a state known as self-tolerance. When a normally functioning immune system attacks
a nonself molecule, the system has the ability to “remember” the specifics of the foreign body. Upon subsequent
encounters with the same species of molecules, the immune system reacts accordingly. With the possible excep-
tion of antibodies passed during lactation, this so called immune system memory is not inherited. Despite the
occurrence of a virus in your family, your immune system must “learn” from experience with the many millions
of distinctive nonself molecules in the sea of microbes in which we live. Learning entails producing the appro-
priate molecules and cells to match up with and counteract each nonself invader.
Any substance capable of triggering an immune response is called an antigen. Antigens are not to be con-
fused with allergens, which are most often harmless substances (such as ragweed pollen or cat hair) that provoke
the immune system to set off the inappropriate and harmful response known as allergy. An antigen can be a virus,
a bacterium, a fungus, a parasite, or even a portion or product of one of these organisms. Tissues or cells from
another individual (except an identical twin, whose cells carry identical self-markers) also act as antigens; because
the immune system recognizes transplanted tissues as foreign, it rejects them. The body will even reject nourish-
ing proteins unless they are first broken down by the digestive system into their primary, nonantigenic building
blocks. An antigen announces its foreignness by means of intricate and characteristic shapes called epitopes, which
protrude from its surface. Most antigens, even the simplest microbes, carry several different kinds of epitopes on
their surface; some may even carry several hundred. Some epitopes will be more effective than others at stimu-
lating an immune response. Only in abnormal situations does the immune system wrongly identify self as non-
self and execute a misdirected immune attack. The result can be a so-called autoimmune disease such as
rheumatoid arthritis or systemic lupus erythematosis. The painful side effects of these diseases are caused by a per-
son’s immune system actually attacking itself.