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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 (germs) that
can cause infection, disease, and
even death. The immune system also gets rid
of abnormal cancerous cells that are
growing out of control. When functioning
properly, it fights off infection
and keeps you healthy. But when it
malfunctions, germs that enter the
body can more easily cause disease or death. Some important
components of the immune system are:
•
Dendritic cells and macrophages
• T cells
• Killer T
cells
• Natural
killer cells
• B cells
• Neutrophils
Dendritic Cells and
Macrophages
Dendritic cells and
macrophages are the immune system's first line of
defense. Dendritic
cells are found mostly in the skin and mucous
membranes that protect
the openings of the body (e.g., the 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.
These two types of
white blood cells are known as scavengers. They
engulf (eat) foreign
invaders, break them apart, and display pieces of
the intruders—known
as antigens—on their surfaces. They 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
Killer T cells
directly attack and destroy (kill) cells infected by viruses as
well as abnormal cancerous
cells. Yet another type of T cell, called
suppressor T cells, calls off the
immune system attack once the invader
is conquered. (This is to make sure
the killer cells don't go overboard, and relax 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 engulf
and destroy infected cells and abnormal cancerous cells.
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 immunoglobutins). Antibodies are proteins that
attach to antigens like a key fits a lock. Each antibody matches a
specific antigen.
When an antibody matches up with an antigen, it has in
essence marked the intruders for
destruction by scavenger immune
cells. Antibodies also activate a
complex chemical chain reaction, called the complement system.
This system's purpose is to destroy bacteria,
which it essentially does by punching
holes in bacterial membranes (or thin walls), which kills them.
When you are exposed
to a pathogen for the first time, it usually takes awhile (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) lingering in your body that recognize the repeat
invader. This allows the immune system to go into action right
away. This also is why people get some diseases, such as chickenpox or
measles, only once. And, this is the basis for how vaccines work—they
cause your immune system to produce antibodies even though you have not
actually had the disease. This is why the expression "vaccinated
against" some disease (e.g., smallpox) is used.
Neutrophils
Neutrophils are another type of white blood cell. They are made in the
bone marrow. When they
are needed to fight infection, they leave the
marrow and travel anywhere in the
body to fight it. These cells are your main defense against
bacteria. They eat bacteria and produce toxic chemicals that destroy
them.
Key Organs of the Immune System
The immune system has
special organs, called lymph organs, that serve as a home base for all
of these white blood cells. They are spread throughout the body. Lymph
organs include the bone marrow and the thymus, as well as the
lymph nodes, spleen, tonsils and
adenoids, the appendix, and clumps of tissue in the small
intestine known as Peyer's patches. (Some people would also consider the
blood and vessels that carry these blood cells to and from the other
structures to be lymph organs.)
Lymph nodes are located along the so-called lymphatic routes. There are
nodes, or clusters, in
the neck, armpits, abdomen, and groin. Each lymph node contains B cells,
and T cells, and other cells, ready to fight invaders.
The spleen is a very 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 most important roles in the immune defense it to help
the body weed out and discard worn-out white blood cells. It also houses
various white blood cells, waiting for instruction to go out and fight
infection.
The lymphatic vessels carry lymph,
which is a clear fluid that "bathes" the
body's tissues, helping to clean out
invaders or germs. The vessels carry the
fluid to the lymph nodes, which can
sort out the antigens in order to begin the fight against them.
HIV and the Immune System
In HIV+ people, the
virus attacks the CD4 cells that coordinate the immune
response. This causes the CD4 cells
to lose the ability to communicate with the rest of the immune
system. And without CD4 cells organizing the rest of the immune system,
important immune cells don't know which invaders need to be
removed from the body. When this
coordination breaks down, people are at risk for
opportunistic infections (Ol's) 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 mutates so rapidly that it can change
faster than the immune system can respond to it.
Other problems may
result from suppression of the bone marrow, which can
occur as a side effect of certain
HIV drugs like AZT (Retrovir). The bone marrow is where immune
cells are produced, so when it's suppressed, you may
have lower numbers of immune cells
available, which again may cause you to be a little more
vulnerable to infections.
Effective combination
HIV treatment can stop the virus from replicating
(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. Researchers are also studying new HIV
therapies that
they hope will help repair the damaged immune system and
restore lost immune function.
CD4 and CD8 cells
The Basics
CD4 and CD8 cells are
white blood cells that play important roles in your body's
immune
response.
Tests that count your CD4 and CD8 cells provide a picture of
your immune
system health. Along with your viral load, your CD4 cell
count can help your doctor tell whether
your HIV disease is progressing or not, and how
well your HIV drugs
are working.
• CD4 cells (sometimes
called T-helper cells)
These white blood
cells help coordinate the various activities of your
immune system. HIV
targets CD4 cells more than any other kind of cell
in your body. A
normal CD4 cell count is about 600-1,500 cells. CD4 cell
counts are often
slightly higher in HIV+ women compared to HIV+ men
(viral load in HIV+
women also tends to be slightly higher, relative to men, at the same
stage of disease). CD4 cell counts usually fall as HIV
disease progresses.
• CD8 cells
(T-suppressor or killer T cells)
There are two main
types of CD8 cells. T-suppressor cells inhibit or
suppress immune
responses. Killer T cells attack ("kill") cancerous cells and cells
infected with viruses. A normal CD8 cell count is about 3001,000
cells. CD8 cell counts usually rise over time in HIV+ people, but
why and how
these increases relate to the health of your immune
system is not well understood.
CD4 Count and HIV
Without HIV
medication, the virus infects more and more cells. This causes the
CD4
count to decrease, usually by about 30 to 100 cells per year, in most
HIV+ people. As the CD4 count
goes down, an HIV+ person becomes more likely to develop
opportunistic infections (Ol's) and cancers.
• Above 500: People with CD4 counts above 500 cells usually have fairly
normal immune function and are at low risk
for OI's
• Below 350: Current
U.S. government guidelines recommend that you
should consider HIV treatment when your
CD4 count falls below 350 cells
• Below 200: A person with a CD4 cell count below 200
cells is diagnosed
as having AIDS.
The guidelines recommend starting treatment at this point, if you have
not already done so
People with low CD4
counts are prone to developing Ols such as Pneumocystis carinii
pneumonia (PCP), Mycobacterium avium complex (MAC), and cytomegalovirus
(CMV). As CD4 cells drop below 200, your doctor will recommend
medications to prevent these infections.
Many. people have
dramatic CD4 cell increases when they start effective HIV
treatment. If the drugs succeed in
slowing or stopping HIV replication, fewer new CD4 cells will be
infected and the CD4 count can recover—the "proof" of
which you see by the increasing
numbers. But the CD4 count can also fall again
if you stop taking your drugs
correctly, or if your HIV becomes resistant to the
drugs. So, your CD4 count is a very
valuable tool for monitoring your HIV disease progression and how
well your HIV medicines are working.
The U.S. government treatment guidelines now recommend
that people
consider starting HIV treatment when their CD4 cell
counts fall below 350 cells.
The recommended level for starting treatment used to be
500 cells. This
change was made because researchers found that starting
treatment with more
than 350 but less
than 500 CD4 cells did not add much benefit.
CD4 and CD8 Cell
Tests
CD4 and CD8 cell
tests are simple blood tests ordered by your doctor. When
you are first diagnosed as HIV+ or
when you first start treatment, you should
get "baseline" CD4 and CD8 cell
tests. Baseline tests give a current picture of your immune
system, when you first enter a doctor's care. Later tests can be
compared against these first results
to see how things are changing over time and with treatment.
You should get your CD4 cell count checked about every three to six
months--or as often as
your doctor recommends. You may need more frequent CD4 cell
tests if your count is low or
falling, or if you are starting or changing treatment.
Many factors can affect your CD4 cell count, including the time of day,
stress, your menstrual
cycle, and infections such as the flu. If you get a result or
number back that surprises you or
your doctor, your doctor will probably want you to get a second
test. That second test would confirm any unexpected results or prove
that those results were random, sort of a fluke (that is, not
significant). Don't worry too much about a single abnormal test result;
trends
Preventing and Treating Ols
The best way to
prevent Ols is to keep your immune system as strong as
possible by taking
HIV drugs before your CD4 cell count falls too low (below
200). This allows the immune system to do its job of
controlling infections.
If your CD4 cells do
fall below 200, taking appropriate medication at certain
CD4 cell levels can
prevent many Ols from developing. Taking medication to
prevent disease is called "prophylaxis."
Effective treatment options are available in most cases if you do
develop an 01. After you recover, you may still need to receive on-going
maintenance treatment to prevent the 01
from coming back.
You may be able to stop prophylaxis or maintenance treatments if your
CD4 cell
count goes up. You should not discontinue any treatment without
discussing it first with your doctor.
Ols and Women
There are certain sex differences in Ols:
• Men are eight times more likely than women to develop Kaposi's sarcoma
(KS)
• Women are more
likely than men to develop bacterial pneumonia
• Women may have higher rates of herpes simplex infections than men
Dysplasia is a
pre-cancerous condition in the female reproductive system, It is often
more severe and difficult to treat in HIV+ women than in HIV-negative
women.
Untreated dysplasia can lead to cervical cancer, a life-threatening
illness and an AIDS-defining condition.
It is important for
HIV+ women to have regular Pap smears and gynecological
exams
to identify infection,
dysplasia, or cancer. In addition, HIV+ women
should have regular lab tests and
appointments with their HIV provider to
monitor their health. On-going
medical care allows for the effective prevention or early
diagnosis and treatment of Ols.
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