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
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....