The Immune System

Essay by Eddie1High School, 11th gradeA+, October 1996

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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...