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Bacteria and VirusesYour body is a multi-cellular
organism made up of perhaps 100 trillion cells. The cells in your body are
fairly complicated machines. Each one has a nucleus, energy production
equipment, etc. Bacteria are single-celled organisms that are much simpler. For
example, they have no nucleus. They are perhaps 1/100th the size of a human cell
and might measure 1 micrometer long. Bacteria are completely independent
organisms able to eat and reproduce - they are sort of like fish swimming in the
ocean of your body. Under the right conditions bacteria reproduce very quickly:
One bacteria divides into two separate bacteria perhaps once every 20 or 30
minutes. At that rate, one bacteria can become millions in just a few hours.
A virus is a different breed altogether. A virus is not really alive. A virus
particle is nothing but a fragment of DNA in a protective coat. The virus comes
in contact with a cell, attaches itself to the cell wall and injects its DNA
(and perhaps a few enzymes) into the cell. The DNA uses the machinery inside the
living cell to reproduce new virus particles. Eventually the hijacked cell dies
and bursts, freeing the new virus particles; or the viral particles may bud off
of the cell so it remains alive. In either case, the cell is a factory for the
virus. |
Components
of the Immune System
One of the funny things about the immune system
is that it has been working inside your body your entire life but you probably
know almost nothing about it. For example, you are probably aware that inside
your chest you have an organ called a "heart". Who doesn't know that they have a
heart? You have probably also heard about the fact that you have lungs and a
liver and kidneys. But have you even heard about your thymus? There's a good
chance you don't even know that you have a thymus, yet its there in your chest
right next to your heart. There are many other parts of the immune system that
are just as obscure, so let's start by learning about all of the parts.
The most obvious part of the immune system is what you can see. For example,
skin is an
important part of the immune system. It acts as a primary boundary between germs
and your body. Part of your skin's job is to act as a barrier in much the same
way we use plastic wrap to protect food. Skin is tough and generally impermeable
to bacteria and viruses. The epidermis contains special cells called Langerhans
cells (mixed in with the melanocytes in the
basal layer) that are an important early-warning component in the immune system.
The skin also secretes antibacterial substances. These substances explain why
you don't wake up in the morning with a layer of mold growing on your skin --
most bacteria and spores that land on the skin die quickly.
Your nose, mouth and eyes are also obvious entry points for germs. Tears and
mucus contain an enzyme (lysozyme) that breaks down the cell wall of many
bacteria. Saliva is also anti-bacterial. Since the nasal passage and lungs are
coated in mucus, many germs not killed immediately are trapped in the mucus and
soon swallowed. Mast cells also line the nasal passages, throat, lungs and skin.
Any bacteria or virus that wants to gain entry to your body must first make it
past these defenses.
Once inside the body, a germ deals with the immune system at a different
level. The major components of the immune system are:
- Thymus
- Spleen
- Lymph system
- Bone marrow
- White blood cells
- Antibodies
- Complement system
- Hormones
Let's look at each of these components in detail.
Lymph
System
The lymph system is most familiar to people because doctors and
mothers often check for "swollen lymph nodes" in the neck. It turns out that the
lymph nodes are just one part of a system that extends throughout your body in
much the same way your blood vessels do. The main difference between the blood
flowing in the circulatory system and the lymph flowing in the lymph system is
that blood is pressurized by the heart, while the lymph system is passive. There
is no "lymph pump" like there is a "blood pump" (the heart). Instead, fluids
ooze into the lymph system and get pushed by normal body and muscle motion to
the lymph nodes. This is very much like the water and sewer systems in a
community. Water is actively pressurized, while sewage is passive and flows by
gravity.
Lymph is a clearish liquid that bathes the cells with water and nutrients.
Lymph is blood plasma -- the liquid that makes up blood minus the red and white
cells. Think about it -- each cell does not have its own private blood vessel
feeding it, yet it has to get food, water, and oxygen to survive. Blood
transfers these materials to the lymph through the capillary walls, and lymph
carries it to the cells.
The cells also produce proteins and waste products and the lymph absorbs these
products and carries them away. Any random bacteria that enter the body also
find their way into this inter-cell fluid. One job of the lymph system is to
drain and filter these fluids to detect and remove the bacteria. Small lymph
vessels collect the liquid and move it toward larger vessels so that the fluid
finally arrives at the lymph nodes for processing.
Lymph nodes contain filtering tissue and a large number of lymph cells. When
fighting certain bacterial infections, the lymph nodes swell with bacteria and
the cells fighting the bacteria, to the point where you can actually feel them.
Swollen lymph nodes are therefore a good indication that you have an infection
of some sort.
Once lymph has been filtered through the lymph nodes it re-enters the
bloodstream.
Thymus
The thymus lives in your chest, between your breast
bone and your heart. It is responsible for producing T-cells (see the next
section), and is especially important in newborn babies - without a thymus a
baby's immune system collapses and the baby will die. The thymus seems to be
much less important in adults - for example, you can remove it and an adult will
live because other parts of the immune system can handle the load. However, the
thymus is important, especially to T cell maturation (as we will see in the
section on white blood cells below).
Spleen
The spleen filters the blood
looking for foreign cells (the spleen is also looking for old red blood cells in
need of replacement). A person missing their spleen gets sick much more often
than someone with a spleen.
Bone marrow
Bone marrow produces new
blood cells, both red and white. In the case of red blood cells the cells are
fully formed in the marrow and then enter the bloodstream. In the case of some
white blood cells, the cells mature elsewhere. The marrow produces all blood
cells from stem cells. They are called "stem cells" because they can
branch off and become many different types of cells - they are precursors to
different cell types. Stem cells change into actual, specific types of white
blood cells.
White blood cells
White blood cells are
described in detail in the next section.
Antibodies
Antibodies (also referred to
as immunoglobulins and gammaglobulins) are produced by white blood cells. They
are Y-shaped proteins that each respond to a specific antigen (bacteria,
virus or toxin). Each antibody has a special section (at the tips of the two
branches of the Y) that is sensitive to a specific antigen and binds to it in
some way. When an antibody binds to a toxin it is called an antitoxin (if the
toxin comes from some form of venom, it is called an antivenin). The binding
generally disables the chemical action of the toxin. When an antibody binds to
the outer coat of a virus particle or the cell wall of a bacterium it can stop
their movement through cell walls. Or a large number of antibodies can bind to
an invader and signal to the complement system that the invader needs to be
removed.
Antibodies come in five classes:
- Immunoglobulin A (IgA)
- Immunoglobulin D (IgD)
- Immunoglobulin E (IgE)
- Immunoglobulin G (IgG)
- Immunoglobulin M (IgM)
Whenever you see an abbreviation like IgE
in a medical document, you now know that what they are talking about is an
antibody.
For additional information on antibodies see The
Antibody Resource Page.
Complement
System
The complement system, like antibodies, is a series of
proteins. There are millions of different antibodies in your blood stream, each
sensitive to a specific antigen. There are only a handful of proteins in the
complement system, and they are floating freely in your blood. Complements are
manufactured in the liver. The complement proteins are activated by and work
with (complement) the antibodies, hence the name. They cause lysing (bursting)
of cells and signal to phagocytes that a cell needs to be removed.
For additional information on complements, see The
Complement System.
Hormones
There are several hormones
generated by components of the immune system. These hormones are known generally
as lymphokines. It is also known that certain hormones in the body
suppress the immune system. Steroids and corticosteroids (components of
adrenaline) suppress the immune system.
Tymosin (thought to be produced by the thymus) is a hormone that encourages
lymphocyte production (a lymphocyte is a form of white blood cell - see below).
Interleukins are another type of hormone generated by white blood cells. For
example, Interleukin-1 is produced by macrophages after they eat a foreign cell.
IL-1 has an interesting side-effect - when it reaches the hypothalamus it
produces fever and fatigue. The raised temperature of a fever is known to kill
some bacteria.
For additional information see Manifestations
of Infection: Fever and IL-1.
Tumor Necrosis Factor
Tumor Necrosis
Factor (TNF) is also produced by macrophages. It is able to kill tumor cells,
and it also promotes the creation of new blood vessels so it is important to
healing.
Interferon
Interferon interferes with
viruses (hence the name) and is produced by most cells in the body. Interferons,
like antibodies and complements, are proteins, and their job is to let cells
signal to one another. When a cell detects interferon from other cells, it
produces proteins that help prevent viral replication in the cell.
White
Blood Cells
You are probably aware of the fact that you have "red
blood cells" and "white blood cells" in your blood. The white blood cells are
probably the most important part of your immune system. And it turns out that
"white blood cells" are actually a whole collection of different cells that work
together to destroy bacteria and viruses. Here are all of the different types,
names and classifications of white blood cells working inside your body right
now:
- Leukocytes
- Lymphocyte
- Monocytes
- Granulocytes
- B-cells
- Plasma cells
- T-cells
- Helper T-cells
- Killer T-cells
- Suppressor T-cells
- Natural killer cells
- Neutrophils
- Eosinophils
- Basophils
- Phagocytes
- Macrophages
Leukocytes
Learning all of these different names and the
function of each cell type takes a bit of effort, but you can understand
scientific articles a lot better once you get it all figured out! Here's a quick
summary to help you get all of the different cell types organized in your brain.
All white blood cells are known officially as leukocytes. White blood
cells are not like normal cells in the body -- they actually act like
independent, living single-cell organisms able to move and capture things on
their own. White blood cells behave very much like amoeba in their movements and
are able to engulf other cells and bacteria. Many white blood cells cannot
divide and reproduce on their own, but instead have a factory somewhere in the
body that produces them. That factory is the bone marrow.
Leukocytes are divided into three classes:
- Granulocytes - Granulocytes make up 50% to 60% of all leukocytes.
Granulocytes are themselves divided into three classes: neutrophils, eosinophils
and basophils. Granulocytes get their name because they contain granules, and
these granules contain different chemicals depending on the type of cell.
- Lymphocyte - Lymphocytes make up 30% to 40% of all leukocytes. Lymphocytes
come in two classes: B cells (those that mature in bone marrow) and T cells
(those that mature in the thymus).
- Monocyte - Monocytes make up 7% or so of all leukocytes. Monocytes evolve
into macrophages.
All white blood cells start in bone marrow as stem cells. Stem cells
are generic cells that can form into the many different types of leukocytes as
they mature. For example, you can take a mouse, irradiate it to kill off
its bone marrow's ability to produce new blood cells, and then inject stem cells
into the mouse's blood stream. The stem cells will divide and differentiate into
all different types of white blood cells. A "bone marrow transplant" is
accomplished simply by injecting stem cells from a donor into the blood stream.
The stem cells find their way, almost magically, into the marrow and make their
home there. (See the Blood
& Marrow Transplantation FAQ to learn more about marrow transplants.)
Each of the different types of white blood cells have a special role in the
immune system, and many are able to transform themselves in different ways. The
following descriptions help to understand the roles of the different cells.
- Neutrophils are by far the most common form of white blood cells that
you have in your body. Your bone marrow produces trillions of them every day and
releases them into the bloodstream, but their life span is short -- generally
less than a day. Once in the bloodstream neutrophils can move through capillary
walls into tissue. Neutorphils are attracted to foreign material, inflammation
and bacteria. If you get a splinter or a cut, neutrophils will be attracted by a
process called chemotaxis. Many single-celled organisms use this same process --
chemotaxis lets motile cells move toward higher concentrations of a chemical.
Once a neutrophil finds a foreign particle or a bacteria it will engulf it,
releasing enzymes, hydrogen peroxide and other chemicals from its granules to
kill the bacteria. In a site of serious infection (where lots of bacteria have
reproduced in the area), pus will form. Pus is simply dead neutrophils and other
cellular debris.
- Eosinophils and basophils are far less common than neutrophils.
Eosinophils seem focused on parasites in the skin and the lungs, while Basophils
carry histamine and therefore important (along with mast cells) to causing
inflammation. From the immune system's standpoint inflammation is a good thing.
It brings in more blood and it dilates capillary walls so that more immune
system cells can get to the site of infection.
- Of all blood cells, macrophages are the biggest (hence the name
"macro"). Monocytes are released by the bone marrow, float in the bloodstream,
enter tissue and turn into macrophages. Most boundary tissue has its own devoted
macrophages. For example, alveolar macrophages live in the lungs and keep the lungs
clean (by ingesting foreign particles like smoke and dust) and disease free (by
ingesting bacteria and microbes). Macrophages are called langerhans cells when
they live in the skin. Macrophages also swim freely. One of their jobs is to
clean up dead neutrophils -- macropghages clean up pus, for example, as part of
the healing process.
- The lymphocytes handle most of the bacterial and viral infections
that we get. Lymphocytes start in the bone marrow. Those destined to become B
cells develop in the marrow before entering the bloodstream. T cells start in
the marrow but migrate through the bloodstream to the thymus and mature there. T
cells and B cells are often found in the bloodstream but tend to concentrate in
lymph tissue such as the lymph nodes, the thymus and the spleen. There is also
quite a bit of lymph tissue in the digestive system. B cells and T cells have
different functions.
- B cells, when stimulated, mature into plasma cells -- these are the
cells that produce antibodies. A specific B cell is tuned to a specific germ,
and when the germ is present in the body the B cell clones itself and produces
millions of antibodies designed to eliminate the germ.
- T cells, on the other hand, actually bump up against cells and kill
them. T cells known as Killer T cells can detect cells in your body that are
harboring viruses, and when it detects such a cell it kills it. Two other types
of T cells, known as Helper and Suppressor T cells, help sensitize killer T
cells and control the immune response.
Helper T cells are actually quite important and interesting. They are
activated by Interleukin-1, produced by macrophages. Once activated, Helper T
cells produce Interleukin-2, then interferon and other chemicals. These
chemicals activate B cells so that they produce antibodies. The complexity and
level of interaction between neutrophils, macrophages, T cells and B cells is
really quite amazing.
Because white blood cells are so important to the immune system, they are
used as a measure of immune system health. When you hear that someone has a
"strong immune system" or a "suppressed immune system", one way it was
determined was by counting different types of white blood cells in a blood
sample. A normal white blood cell count is in the range of 4,000 to 11,000 cells
per microliter of blood. 1.8 to 2.0 helper T-cells per suppressor T-cell is
normal. A normal absolute neutrophil count (ANC) is in the range of 1,500 to
8,000 cells per microliter. An article like Introduction
to Hematology can help you learn more about white blood cells in general and
the different types of white blood cells found in your body.
One important question to ask about white blood cells (and several other
parts of the immune system) is, "How does a white blood cell know what to attack
and what to leave alone? Why doesn't a white blood cell attack every cell in the
body?" There is a system built into all of the cells in your body called the
Major Histocompatibility Complex (MHC) (also known as the Human Leukocyte
Antigen (HLA)) that marks the cells in your body as "you". Anything that the
immune system finds that does not have these markings (or that has the wrong
markings) is definitely "not you" and is therefore fair game. Encyclopedia
Britannica has this to say about the MHC:
"There are two major types of MHC protein molecules--class I
and class II--that span the membrane of almost every cell in an organism. In
humans these molecules are encoded by several genes all clustered in the same
region on chromosome 6. Each gene has an unusual number of alleles (alternate
forms of a gene). As a result, it is very rare for two individuals to have the
same set of MHC molecules, which are collectively called a tissue type.
MHC molecules are important components of the immune response. They allow
cells that have been invaded by an infectious organism to be detected by cells
of the immune system called T lymphocytes, or T cells. The MHC molecules do this
by presenting fragments of proteins (peptides) belonging to the invader on the
surface of the cell. The T cell recognizes the foreign peptide attached to the
MHC molecule and binds to it, an action that stimulates the T cell to either
destroy or cure the infected cell. In uninfected healthy cells the MHC molecule
presents peptides from its own cell (self peptides), to which T cells do not
normally react. However, if the immune mechanism malfunctions and T cells react
against self peptides, an autoimmune disease arises." See Biology
of the Immune System and Major
Histocompatibility Complex for additional details.
Using This
knowledge
Now that you have taken the time to learn all of these facts
about the immune system, you can understand many things about the immune system
in a whole new light. The following sections guide you through several topics
that are all related to the immune system.
Vaccinations
There are many diseases
that, if you catch them once, you will never catch again. Measles is a good
example, as is chicken pox. What happens with these diseases is that they make
it into your body and start reproducing. The immune system gears up to eliminate
them. In your body you already have B cells that can recognize the virus and
produce antibodies for it. However, there are only a few of these cells for each
antibody. Once a particlular disease is recognized by these few specific B
cells, the B cells turn into plasma cells, clone themselves and start pumping
out antibodies. This process takes time, but the disease runs it course and is
eventually eliminated. However, while it is being eliminated, other B cells for
the disease clone themselves but do not generate antibodies. This second set of
B cells remains in your body for years, so if the disease reappears your body is
able to eliminate it immediately before it can do anything to you.
A vaccine is a weakened form of a disease. It is either a killed form of the
disease, or it is a similar but less virulent strain. Once inside your body your
immune system mounts the same defense, but because the disease is different or
weaker you get few or no symptoms of the disease. Now, when the real disease
invades your body, your body is able to eliminate it immediately.
Vaccines exist for all sorts of diseases, both viral and bacterial: measles,
mumps, whooping cough, tuberculosis, smallpox, polio, typhoid, etc.
Many diseases cannot be cured by vaccines, however. The common cold and
Influenza are two good examples. These diseases either mutate so quickly or have
so many different strains in the wild that it is impossible to inject all of
them into your body. Each time you get the flu, for example, you are getting a
different strain of the same disease.
AIDS
AIDS (Acquired Immune
Deficiency Syndrome) is a disease caused by HIV (the Human Immunodeficiency
Virus). This is a particularly problematic disease for the immune system because
the virus actually attacks immune system cells. In particular, it reproduces
inside Helper T cells and kills them in the process. Without Helper T cells to
orchestrate things, the immune system eventually collapses and the victim dies
of some other infection that the immune system would normally be able to handle.
See How AIDS Works as
well as the links below for more information.
How
Antibiotics Work
Sometimes your immune system is not able to activate
itself quickly enough to outpace the reproductive rate of a certain bacteria, or
the bacteria is producing a toxin so quickly that it will cause permanent damage
before the immune system can eliminate the bacteria. In these cases it would be
nice to help the immune system by killing the offending bacteria directly.
Antibiotics work on bacterial infections. Antibiotics are chemicals that kill
the bacteria cells but do not affect the cells that make up your body. For
example, many antibiotics interrupt the machinery inside bacterial cells that
builds the cell wall. Human cells do not contain this machinery, so they are
unaffected. Different antibiotics work on different parts of bacterial
machinery, so each one is more or less effective on specific types of bacteria.
You can see that, because a virus is not alive, antibiotics have no effect on a
virus.
One problem with antibiotics is that they lose effectiveness over time. If
you take an antibiotic it will normally kill all of the bacteria it targets over
the course of a week or 10 days. You will feel better very quickly (in just a
day or two) because the antibiotic kills the majority of the targeted bacteria
very quickly. However, on occasion one of the bacterial offspring will contain a
mutation that is able to survive the specific antibiotic. This bacteria will
then reproduce and the whole disease mutates. Eventually the new strain is
infecting everyone and the old antibiotic has no effect on it. This process has
become more and more of a problem over time and has become a significant concern
in the medical community.
Immune System Mistakes
Sometimes the
immune system makes a mistake. One type of mistake is called
autoimmunity: the immune system for some reason attacks your own body in
the same way it would normally attack a germ. Two common diseases are caused by
immune system mistakes. Juvenile-onset diabetes is caused by the immune system
attacking and eliminating the cells in the pancreas that produce insulin.
Rheumatoid arthritis is caused by the immune system attacking tissues inside the
joints.
Allergies are another form of immune system error. For some reason, in people
with allergies, the immune system strongly reacts to an allergen that should be
ignored. The allergen might be a certain food, or a certain type of pollen, or a
certain type of animal fur. For example, a person allergic to a certain pollen
will get a runny nose, watery eyes, sneezing, etc. This reaction is caused
primarily by mast cells in the nasal passages. In reaction to the pollen the
mast cells release histamine. Histamine has the effect of causing inflammation,
which allows fluid to flow from blood vessels. Histamine also causes itching. To
eliminate these symptoms the drug of choice is, of course, an antihistamine.
The last example of an immune system mistake is the effect the immune system
has on transplanted tissue. This really isn't a mistake, but it makes organ and
tissue transplants nearly impossible. When the foreign tissue is placed inside
your body, its cells do not contain the correct identification. Your immune
system therefore attacks the tissue. The problem cannot be prevented, but can be
diminished by carefully matching the tissue donor with the recipient and by
using immunosuppressing drugs to try to prevent an immune system reaction. Of
course, by suppressing the immune system these drugs open the patient to
opportunistic infections.
For more information on the immune system and related topics, check out the
links on the next page.
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