Learning About The Immune System
This section of Leukine.com is intended to provide
you with a general understanding of the cells of the immune system and
the role they play in fighting infections. Once you have reviewed the
information here, you should consider visiting the pages on the site
that discuss what cancer therapy does to the immune system.
How the Immune System Works
Your immune system is a complex network of cells that work together to
protect your body against "foreign" or invading cells, including
abnormal cells that can lead to cancer. Most foreign invaders are germs
— infection-causing organisms that you may come into contact with, such
as bacteria, viruses, and fungi. Your immune cells are constantly on guard and ready to go into action to detect and destroy these invaders.
Your immune system can recognize millions of different enemies and
produce secretions and cells to wipe out each of them. The secret to
your immune system's success is its complex and exciting communications
network. Tens of millions of cells, organized into groups, pass
information back and forth, much like bees do when they swarm. When one
group of immune cells discovers the presence of foreign invaders, it
undergoes key changes and produces powerful chemical substances. These
substances allow the cells to regulate their own growth and function,
enlist other immune cells, and direct them to attack the foreign
invaders as well.
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How Immune Cells Distinguish Between Foreign Cells and Normal Cells
Your immune system has an amazing ability to distinguish between your body's own cells (referred to as "self") and foreign cells ("nonself"). Each cell carries protein markers called antigens that identify it as self or nonself. Your body's immune system coexists peacefully with all cells that carry the self marker (except in the case of rare autoimmune diseases where the body mistakes self for nonself).
Anything that carries a nonself marker triggers a response from your immune system. This includes bacteria, fungi,viruses, transplanted tissues or cells from another person (except an identical twin), and abnormal cells that can lead to cancer.
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Key Immune System Cells and Their Functions
All immune cells are white blood cells (WBCs) that arise from a single pluripotent stem cell, which is found in bone marrow. When a pluripotent stem cell divides, it forms two daughter cells. These either remain pluripotent stem cells and replenish the pluripotent stem cell pool, or they become specialized immune cells with dedicated functions. Some of these immune cells will become part of the myeloid lineage, which generates cells such as neutrophils, monocytes/macrophages, and dendritic cells. Others will become part of the lymphoid lineage, which gives rise to T cells and B cells.
Your immune system stockpiles a huge arsenal of cells. Some immune
cells can destroy many foreign invaders, while others are activated to
attack and destroy only specific targets. In order for your immune
system to work effectively, your immune cells must not only
communicate, but also cooperate with each other. In the following pages
we will identify the key cells of the immune system and describe how
they work together to fight infection.
Immune Cell Lineage
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Neutrophils—
Neutrophils
are the most abundant type of WBC and are the first immune system cells to rush to the site of an infection. Neutrophils are known as phagocytes, or "cell eaters," because they engulf foreign organisms and use prepackaged chemicals to destroy them. Although neutrophils are rapidly produced by your immune system, they live and work for a relatively short period of time — usually just a few days.
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Monocytes and macrophages—
Monocytes and macrophages are related to neutrophils and are also phagocytes, which destroy foreign organisms by ingesting them. Monocytes are produced by the bone marrow and released into the bloodstream. Some monocytes enter your tissues, then enlarge and mature into macrophages, or "big eaters." Macrophages
play many important roles within your immune system. As scavengers,
they rid your body of worn-out cells and other debris. They also
secrete a wide range of powerful, antigen-destroying chemicals and work to "activate" other immune cells through a process called antigen presentation (which is described later in this brochure).
Although they are not as abundant as neutrophils, monocytes and macrophages can recognize and destroy more types of foreign invaders, including viruses. Also, while neutrophils have a very short lifespan, macrophages
can remain in the tissues for months or even years. They act as
lookouts, watching for invaders. When necessary they can also direct
other immune cells, such as neutrophils, to the site of an infection.
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Dendritic Cells—
Although dendritic cells
are the least abundant type of WBC, they are the most potent, and are
found in your blood and in most tissues throughout your body. Dendritic cells
are regulators or "coaches" of the immune system. They act as observers
and messengers who go to the battle site, collect pertinent
information, and pass it along.
Dendritic cells
have long, fingerlike projections that increase their size, improve
their mobility to move around, and increase their ability to find and
sample more antigens. They collect information about the infection or invasion by identifying the type and the location of the invader.
Dendritic cells are unique because they are capable of capturing and absorbing many types of foreign antigens. Once they capture the antigens, dendritic cells migrate to the lymph nodes, where they use the captured antigens to "activate" large numbers of T cells through antigen presentation. This activation process tells the T cells how to respond. Thus, dendritic cells dictate how other parts of the immune system should react. Dendritic cells are often called "professional" antigen-presenting cells because they do it better than any other type of immune cell.
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T cells—
There are two main kinds of T cells: helper T cells and cytotoxic T cells (also known as killer T cells). Helper T cells
direct and regulate your body's immune responses, while killer T cells
directly attack cells that are infected or cancerous. In order for T cells to do their job, however, they must be "activated" by other immune cells, such as macrophages and dendritic cells.
Upon activation, T cells mature to become either killer T cells or helper T cells.
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B cells—
B cells produce antibodies, which are proteins that bind to antigens and mark them for destruction. B cells are activated when a T cell presents an antigen to them. The B cells then begin producing antibodies that bind specifically to that antigen, which is similar to the way a key matches a lock. Activated B cells also begin multiplying, which further increases the body's immune response.
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How the Immune System Responds to Infection
Once foreign cells enter your body, your immune system fights them
using two armies of cells: nonspecific cells (also known as innate
immunity) and specific cells (also known as adaptive immunity). Some
immune cells have multiple functions and therefore may be considered
both nonspecific and specific. Neutrophils, macrophages, and dendritic cells
are critical immune cells because they attack foreign invaders of all
kinds. These cells are also critical because they circulate
continuously within your body looking for any foreign antigen.
Your body also uses specific immune cells to fight infection. These specific cells can't distinguish between self and nonself and therefore only go into action when nonspecific cells (such as macrophages and dendritic cells) give them information about new invaders through antigen presentation.
Antigen presentation is an amazing and highly effective process. When macrophages and dendritic cells attack and destroy a foreign antigen, they capture a portion of it and then present it to your body's T cells. In doing this, the antigen-presenting cells "activate" certain T cells to aggressively seek out and destroy all foreign invaders that carry that specific antigen. Other T cells can further activate other parts of the adaptive immune system to also destroy the specific foreign antigen.
It is through this and other complex processes that your body's
immune cells work together to destroy foreign invaders and successfully
fight off infection.
Antigen, T cell, B cell, Antibody
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