The Immune System


The immune system is a group of cells, molecules, and tissues that help
defend the body against diseases and other harmful invaders. The immune system
provides protection against a variety of potentially damaging substances that
can invade the body. These substances include disease-causing organisms, such
as bacteria, fungi, parasites, and viruses. The body\'s ability to resist these
invaders is called immunity. A key feature of the immune system is its ability
to destroy foreign invaders while leaving the body\'s own healthy tissues alone.
Sometimes, however, the immune system attacks and damages these healthy tissues.
This reaction is called an autoimmune response or autoimmunity.

The immune system is composed of many parts that work together to fight
infections when pathogens or poisons invade the human body. Pathogens are
disease-causing organisms such as bacteria and viruses. The immune system
reacts to foreign substances through a series of steps know as the immune
response. Any agent perceived as foreign by a body\'s immune system is called an
antigen. Several types of cells may be involved in the immune response to
antigens.

When an antigen enters the body, it may be partly neutralized by
components of the innate immune system. It may be attacked by phagocytes or by
performed antibodies that act together with the complement system. The human
immune system contains approximately 1 trillion T cells and 1 trillion B cells,
located in the lymphoid organs and in the blood, plus approximately 10 billion
antigen-presenting cells located in the lymphoid organs. To maximize the chances
of encountering antigens wherever they may invade the body, lymphocytes
continually circulate between the blood and certain lymphoid tissues. A
lymphocyte spends an average of 30 minutes per day in the blood and recirculates
about 50 times per day between the blood and lymphoid tissues.

Lymphocytes are special types of white blood cells. Like other white
blood cells, lymphocytes originate in the bone marrow, the blood-forming tissue
in the center of many bones. Some lymphocytes mature in the bone marrow and
become B lymphocytes, also know as B cells. The B stands for bone marrow
derived. Some of these cells develop into plasma cells, which produce
antibodies. Antibodies are proteins that attack antigens. They are carried in
the blood, in tears, and in secretions of the nose and the intestines. Other
lymphocytes do not mature in the bone marrow. Instead, they travel through the
bloodstream to the thymus, an organ in the upper chest. In the thymus, the
immature lymphocytes develop into T lymphocytes, also knows as T cells. The T
stands for thymus derived. The B lymphocytes are responsible for the production
of the blood-serum components called immunoglobulins. The T lymphocytes are
responsible for attacking and killing antigens directly. Both T and the B
lymphocytes have the ability to remember previous exposure to a specific antigen,
so that if the same antigen enters the body the T and B lymphocytes can take
faster and better action against it.

There are many disorders that disrupt the immune system\'s operations.
The most serious are the disorders called immunodeficiency diseases, such as
AIDS. These diseases can lead to death. Immunodeficiency diseases are among the
most severe disorders of the immune system. People afflicted with such
conditions lack some basic feature or function of their immune system. As a
result, their immune system fails to respond adequately to harmful invaders.
For this reason, people with immune deficiency diseases suffer from different
kinds of illness.

Allergies are mistaken and harmful responses of the body\'s immune system
to substances that are harmless to most people. The substances that provoke an
allergic reaction are called allergens. They include pollen, dust, mold, and
feathers. Among the common allergic diseases are asthma, eczema, which is an
itchy red swellings of the skin, hay fever, and hives.

If lymphocytes encounter an antigen trapped by the antigen-presenting
cells of the lymphoid organs, lymphocytes with receptors specific to that
antigen stop their migration and settle to mount an immune response locally.
The process of inducing an immune response is called immunization. It may be
either natural, through infection by a pathogen, or artificial, through the use
of serums or vaccines. The immune system cannot protect the body from diseases
by itself. Sometimes it needs help. Physicians give their patients vaccines to
help protect them from certain severe, life-threatening infections. Vaccines
and serums boost the body\'s ability to defend itself against particular types of
viruses or bacteria.

Category: Science