transplanted tissues, and insect venom. Parts of these substances are called anti-
gens,or immunogens. Antigens can be polysaccharides or proteins and provoke
a specific immune response in organisms. Immunityis a specific defensive
response of a host when a foreign substance or organism invades it.
The body reacts to the foreign substance (antigen) by forming antibodies.
Antibodies are proteins that are made by the body in response to an antigen and
can combine specifically with that antigen.
The immune system recognizes a body or substance within the organism as
self or nonself. Self is any body or substance that belongs to the organism. Non-
self is any body or organism that doesn’t belong to the organism. An antigen is
recognized as nonself by the immune system.
An antigen causes the organism to form antibodiesand specialized lympho-
cytesthat target the specific antigen. If the antigen invades again, these antibod-
ies and specialized lymphocytes attack the antigen, making the antigen inactive
or destroyed. This is called an immune responseagainst an antigen.
Cells that become cancerous are recognized as foreign and can be destroyed.
Cancerous cells, once established as a tumor, might have a dreadful effect on the
organism because the immune system is not be able to fight them.ACQUIRED IMMUNITY
The development of antibodies and specialized lymphocytes is called acquired
immunity because it is acquired over an organism’s life through naturally
acquired immunity and artificially acquired immunity means.
Acquired immunityis the protective defense mechanism an organism devel-
ops against foreign substances and microorganisms. This type of immunity is
established throughout an individual’s life.
There are two types of acquired immunity. These are active and passive.
When an individual is exposed to a disease-causing microorganism or foreign
substance, the person’s own immune system responds by making its own anti-
bodies and lymphocytes.
In passive immunity,already-made antibodies are introduced or passed on
to an individual. The individual does not make its own antibodies.
Naturally acquired active immunityoccurs when an individual is exposed to
an infectious disease. The individual’s immune system responds by making its
own antibodies and lymphocytes (T cells and B cells) on its own.
Naturally acquired passive immunityoccurs when antibodies (IgG) are made
by a mother and passed on to the fetus through the placenta. IgA antibodies are(^212) CHAPTER 14 Immunity