antigens more strongly the next time
they enter the body.
Certain white blood cells, called lymphocytes,
are important parts of the
acquired immune system. There are two
different kinds of lymphocytes: B cells
and T cells. B cells release products
called antibodies, which attack antigens.
T cells are divided into helper cells and
killer cells. Helper T cells produce substances
that help new lymphocytes grow.
Killer T cells destroy cells in the body
that are infected by antigens.
Because of acquired immunity, people
get certain diseases, like chicken pox,
only once. Acquired immunity is also
what makes vaccines work.
The spleen is
part of the
human
immune system.
It is a
small organ
that removes
bacteria from
the blood.
Special cells that eat antigens are a part of
natural immunity. B cells that release antibodies
are a part of acquired immunity.
BRITANNICA STUDENT ENCYCLOPEDIA Immune System 119
Problems with the
Immune System
The two types of immunity normally
work together to protect the body from
harmful invaders. But sometimes the
immune system does not work properly.
It may spot and try to destroy cells that
a doctor has put into the body on purpose.
For example, the immune system
may attack a healthy kidney that has
been transplanted into a patient.
The immune system may also overreact
to antigens that are not really harmful
to the body. Allergies are an example of
this type of problem. When some
people breathe in pollen from plants or
eat certain foods, the immune system
attacks the pollen or food particles. The
attack causes sneezing, itching, or other
symptoms.
Another possible problem with the
immune system is called an autoimmune
response. This happens when
lymphocytes see normal, healthy cells as
antigens and attack them. Rheumatoid
arthritis is an example of an autoimmune
disease. In this disease, lymphocytes
attack the joints between bones.
A serious disease of the immune system
is AIDS, or acquired immunodeficiency
syndrome. It is caused by a virus that
attacks the immune system’s helper T
cells. As the T cells die, the immune
system cannot protect the body from
other dangerous infections.
#More to explore
AIDS • Allergy • Vaccine
Impeachment
Impeachment is a process by which a
government official is charged with a
crime. A legislature, or lawmaking body,
handles an impeachment.
In the United States the two houses of
Congress carry out impeachments. First,
the House of Representatives impeaches,
or brings charges against, an official.
When the House impeaches an official,
it does not mean that he or she is guilty
of a crime. It means that there is evidence
that the official may have committed
a crime.
The Senate then holds a trial and acts
as the judge. After hearing the evidence,
the senators vote on whether the official
is guilty or not guilty. Two thirds of the
senators must vote guilty for the official
to be convicted, or found guilty. If
convicted, the official must leave his or
her job.
The impeachment process is rarely used
in the United States. The only U.S.
presidents to be impeached were
Andrew Johnson, in 1868, and Bill
Fever, pain,
redness, and
swelling are
signs that the
immune system
is fighting
antigens.
When a U.S. government official is
impeached the Senate holds a trial to
decide if the official is guilty.
120 Impeachment BRITANNICA STUDENT ENCYCLOPEDIA
Clinton, in 1998. The Senate found
both of them not guilty.
In the United Kingdom the two houses
of Parliament handle impeachments.
The House of Commons impeaches an
official, and the House of Lords serves as
the judge. Impeachment was once fairly
common in England. However, there
have been no impeachments in the
United Kingdom since 1806.
#More to explore
Clinton, Bill • Congress of the United
States • Johnson, Andrew • Legislature
Imperialism
#see Empire.
Inca
The Inca people once ruled a vast
empire in the Andes Mountains of
South America. Their capital was Cuzco,
in what is now Peru. The Inca Empire
included about 12 million people at its
peak in the early 1500s.
How the Inca Lived
The Inca grew such crops as corn,
squash, tomatoes, peanuts, and cotton.
Inca farmers were the first to grow potatoes.
They also raised guinea pigs, ducks,
alpacas, and dogs. The Inca rode animals
called llamas and also used them as work
animals.
The Inca made clothing from llama
wool and cotton. They made pottery
and musical instruments. They also
made things out of metal.
The Inca built their houses using stone
or adobe (sun-baked clay). The ruins of
the Inca city of Machu Picchu are
famous for their well-made buildings.
The Inca spoke a language called Quechua.
They had no form of writing.
However, they kept records using a complicated
system of colored knotted cords
called quipus.
History
According to the traditional belief of the
Inca, they originally came from a village
called Paqari-tampu. In the 1100s they
moved north and settled in Cuzco. In
the early 1400s the Inca began to attack
neighboring peoples and take control of
their lands.Within 100 years the Inca
Empire was at the height of its power.
The Spanish began to explore Peru in
the 1520s. By 1533 Spanish soldiers led
