Submitted on Jun 14, 2016
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Table
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The immune system is made up of cells and organs that protect your body from outside invaders such as bacteria, viruses, fungi, and parasites (types of germs) that can cause infection and disease. The immune system also gets rid of abnormal pre-cancerous cells and cancerous cells that are growing out of control. When it works correctly, it fights off infection and keeps you healthy. However, when it does not work correctly, germs and other abnormal cells in the body can more easily cause disease.
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Key
Organs of the Immune System
The first line of defense against
germs is your skin, the single largest organ of the body. It provides a
physical barrier that keeps bacteria and viruses from entering the body.
Viruses such as HIV cannot get through normal, healthy, unbroken skin. HIV can,
however, get into the body through unbroken mucous membranes, which are the
moist membranes of the vagina (birth canal), rectum, and urethra ('pee hole').
Germs that get inside the body are
taken care of by the internal parts of the immune system. The white blood
cells that defend the body from invaders and get rid of possibly dangerous
abnormal cells begin their lives in the bone marrow. Once they leave the bone
marrow, they travel to the lymph organs, which serve as a home base for mature
white blood cells. There, the white blood cells await instruction to go out and
fight infection.
Lymph organs are spread throughout
the body and include the lymph nodes, thymus, spleen, appendix, tonsils and
adenoids, and clumps of tissue in the small intestine known as Peyer's patches.
Lymph nodes are located in the neck, armpits, abdomen, and groin. Each lymph
node contains cells ready to fight invaders.
The lymphatic vessels connect the
lymph nodes and carry lymph, which is a clear fluid that "bathes" the
body's tissues and helps to clean out invaders or germs.
The spleen is an important organ for
a healthy immune system. It is about the size of a fist, and it is located at
the upper left of the abdomen. One of its key roles is to filter blood and to
identify and get rid of worn-out white blood cells.
Some key cells of the immune system
are:
- Neutrophils
- Dendritic cells and macrophages
- Helper and Killer T cells
- Natural killer cells
- B cells
Neutrophils
Neutrophils are one of the
"first-responders" in the body's response to infection. Normally, you
have more neutrophils than any other type of white blood cell. They
circulate in the bloodstream and are your main defense against bacteria. They
eat bacteria and produce toxic chemicals that destroy them. These are the cells
that give pus its whitish/yellowish color.
Dendritic
Cells and Macrophages
Dendritic cells are found mostly in
the skin and mucous membranes that protect the openings of the body (e.g.,
nose, mouth, and throat). These cells capture and carry invaders to the lymph
nodes or spleen. Macrophages (their name comes from Latin and means "big
eaters") protect different organs, including the intestines, lungs, liver,
and brain. Like dendritic cells, macrophages capture and carry invaders to the
lymph organs.
These two types of white blood cells
are known as scavengers. They engulf (eat) foreign invaders, break them apart,
and display pieces of the germs—known as antigens (from antibody-generating)
—on their surfaces. The body can then make antibodies to that specific germ,
which helps the body get rid of that invader faster and remember it in the
future. These cells also produce chemical messengers (known as cytokines) that
instruct other immune cells to go into action.
T
Cells
Once antigens are processed and
displayed on the surface of macrophages, they can be recognized by helper T
cells (also known as CD4 cells). When CD4 cells "see" the
antigens displayed, they get busy and put the word out to other immune system
cells. In other words, these cells coordinate and direct the activity of other
types of immune cells—such as killer T cells, B cells, and macrophages—calling
them into action to fight the intruder. CD4 cells produce many different
cytokines in order to communicate effectively with other immune system cells.
Killer T cells directly attack and
destroy cells infected by viruses as well as abnormal cells that may become
cancerous. Suppressor T cells call off the immune system attack once the
invader is conquered. This is to make sure the killer T cells stop killing once
their job is done. Both killer T cells and suppressor T cells are also known as
CD8 cells.
Natural
Killer Cells
Natural killer, or NK cells also
destroy infected or abnormal cells. NK cells are an important part of the early
response to viral infections because they kill cells infected by virus while
the body is recruiting killer T cells into action.
B
Cells and Antibodies
B cells are another type of immune
cell that is activated by CD4 cells. When a B cell recognizes an antigen, it
goes into action and produces antibodies (also called immunoglobulins). An
antibody is a protein that attaches to an antigen like a key fits a lock. Each
antibody matches a specific antigen. When an antibody has matched up with an
antigen, it has marked the intruders for destruction by scavenger immune cells.
Antibodies also activate a complex chemical reaction that destroys bacteria by
punching holes in their outside surface (the bacterial membrane), which kills
them.
When you are exposed to a germ for
the first time, it usually takes a while (several weeks to a few months) for
your body to produce antibodies to fight it. But if you were exposed to a germ
in the past, you will usually still have some B cells (called memory cells) in
your body that recognize or 'remember' the repeat invader. This allows the
immune system to go into action right away. This is why people get some
diseases, such as chickenpox or measles, only once. They may get chickenpox or
measles germs in their bodies more than once, but their bodies remember the
germs and react so quickly that they do not get sick.
This is also how vaccines work— they
introduce your body to an inactive form of a particular germ and trigger your
immune system to produce antibodies to that germ. The inactive form prevents
you from getting sick while still allowing your body to make important memory B
cells. This is why the expression "vaccinated against" some disease
(e.g., smallpox) is used.
HIV attacks immune system cells
called CD4 T cells. With a healthy immune system, the
body responds normally to this new infection by producing antibodies to HIV.
However, the antibodies your body creates in response to HIV do not protect you
against HIV like antibodies for other infections. Instead, HIV turns the CD4 T
cells that would kill it into factories for making more copies of itself. This
is why, in the case of HIV, being antibody positive does not mean you are
protected, but rather that HIV is weakening your immune system and using it to
make copies of itself.
As HIV reproduces, it damages or
kills CD4 cells. Over time, the virus decreases both the number and type of CD4
cells. Without CD4 cells organizing the rest of the immune system, important
immune cells do not know which invaders need to be removed from the body. When
the immune response is not properly activated and organized, people are at risk
for opportunistic infections (OIs) and cancers that
usually do not harm people with healthy immune systems.
HIV can also infect macrophages and
other immune cells. Your immune system recognizes and produces antibodies to
HIV, but antibodies alone are not enough to eliminate the virus. This is partly
because HIV changes or mutates faster than the immune system can respond to it.
HIV persists in the body by forming
a "reservoir." The HIV reservoir refers to a collection of inactive,
‘resting,’ or latent HIV-infected cells. Researchers have reported evidence
suggesting that HIV-infected cells can persist and expand by genetically
cloning or copying themselves. In other words, elimination of HIV in the body
(complete cure) will require not only that HIV be eliminated from the
bloodstream, but also that we find a way to prevent these latent cells from
multiplying or empty the reservoirs altogether.
There are several known reservoirs,
including immune cells in the gut, lymphoid tissue, blood, the brain, the
genital tract, and bone marrow. It is unclear when reservoirs are established,
but recent research suggests that it could be as early as 24 hours after
initial infection.
Other problems may result from
limiting the activity of the bone marrow, which can occur as a side effect of certain HIV drugs like Retrovir
(zidovudine). Since the bone marrow is where immune cells are produced, if its
activity is reduced you may have lower numbers of immune cells available, which
again may cause you to be more vulnerable to infections.
The good news is that early
treatment with a combination of HIV drugs can minimize the size of the
reservoir. In addition, newer HIV drugs have fewer side effects and are more
effective at stopping the virus from multiplying (making copies of itself) and
infecting more CD4 cells. Since CD4 cells are key to a healthy immune response,
this can give your immune system a fighting chance to replenish its supply of
CD4 cells and to defend itself (you!) against opportunistic infections.
Read more articles from The Well Project, here.
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