Inside your body there is an amazing protection
mechanism called the immune system. It is designed to defend you
against millions of bacteria, microbes, viruses, toxins and parasites that
would love to invade your body. To understand the power of the immune system,
all that you have have to do is look at what happens to anything once it
dies. That sounds gross, but it does show you something very important
about your immune system. When something dies its immune system (along
with everything else) shuts down. In a matter of hours the body is invaded
by all sorts of bacteria, microbes, parasites... None of these things are
able to get in when your immune system is working, but the moment your
immune system stops the door is wide open. Once you die it only takes a
few weeks for these organisms to completely dismantle your body and carry
it away, until all that's left is a skeleton. Obviously your immune system
is doing something amazing to keep all of that dismantling from happening
when you are alive!
The immune system is complex, intricate
and interesting. And there are at least two good reasons for you to know
more about it. First, it is just plain fascinating to understand where
things like fevers, hives, inflammation, etc. come from when they happen
inside your own body. You also hear a lot about the immune system in the
news as new parts of it are understood and new drugs come on the market
- knowing about the immune system makes these news stories understandable.
In this edition of How
Stuff Works we will take a look at how your immune system works
so that you can understand what it is doing for you each day, as well as
what it is not!
Your Immune System
Your immune system works around the clock
in thousands of different ways, but it does its work largely unnoticed.
One thing that causes us to really notice our immune system is when it
fails for some reason. We also notice it when it does something that has
a side effect we can see or feel. Here are several examples:
When you get a cut, all sorts of bacteria
and viruses enter your body through the break in the skin. When you get
a splinter you also have the sliver of wood as a foreign object inside
your body. Your immune system responds and eliminates the invaders while
the skin heals itself and seals the puncture. In rare cases the immune
system misses something and the cut gets infected. It gets inflamed and
will often fill with pus. Inflammation and pus are both side-effects of
the immune system doing its job.
When a mosquito bites you, you get a red,
itchy bump. That too is a visible sign of your immune system at work.
Each day you inhale thousands of germs (bacteria
and viruses) that are floating in the air. Your immune system deals with
all of them without a problem. Occasionally a germ gets past the immune
system and you catch a cold, get the flu or worse. A cold or flu is a visible
sign that your immune system failed to stop the germ. The fact that you
get over the cold or flu is a visible sign that your immune system was
able to eliminate the invader after learning about it. If your immune system
did nothing, you would never get over a cold or anything else.
Each day you also eat hundreds of germs, and
again most of these die in the saliva or the acid of the stomach. Occasionally,
however, one gets through and causes food poisoning. There is normally
a very visible effect of this breach of the immune system: vomiting and
diarrhea are two of the most common symptoms.
There are also all kinds of human ailments
that are caused by the immune system working in unexpected or incorrect
ways that cause problems. For example, some people have allergies. Allergies
are really just the immune system overreacting to certain stimuli that
other people don't react to at all. Some people have diabetes, which is
caused by the immune system inappropriately attacking cells in the pancreas
and destroying them. Some people have rheumatoid arthritis, which is caused
by the immune system acting inappropriately in the joints. In many different
diseases, the cause is actually an immune system error!
Finally, we sometimes see the immune system
because it prevents us from doing things that would be otherwise beneficial.
For example, organ transplants are much harder than they should be because
the immune system often rejects the transplanted organ.
of the Immune System
Let's start at the beginning. What does
it mean when someone says "I feel sick today?" What is a disease? By understanding
the different kinds of diseases it is possible to see what types of disease
the immune system helps you handle.
When you "get sick", your body is not able
to work properly or at its full potential. There are many different ways
for you to get sick - here are some of them:
Viral and bacterial infections are by far
the most common causes of illness for most people. They cause things like
colds, influenza, measles, mumps, malaria, AIDS and so on. The job of your
immune system is to protect your body from these infections. The immune
system protects you in three different ways:
Mechanical damage. If you break a bone or
tear a ligament you will be "sick" (your body will not be able to perform
at its full potential). The cause of the problem is something that is easy
to understand and visible.
or mineral deficiency. If you do not get enough vitamin D your body
is not able to metabolize calcium properly and you get a disease known
as rickets. People with rickets have weak bones (they break easily) and
deformities because the bones do not grow properly. If you do not get enough
vitamin C you get scurvy, which causes swollen and bleeding gums, swollen
joints and bruising. If you do not get enough iron you get anemia, and
Organ degradation. In some cases an organ
is damaged or weakened. For example, one form of "heart disease" is caused
by obstructions in the blood vessels leading to the heart muscle, so that
the heart does not get enough blood. One form of "liver disease", known
as Cirrhosis, is caused by damage to liver cells (drinking too much alcohol
is one cause).
Genetic disease. A genetic disease is caused
by a coding error in the DNA. The coding error causes too much or too little
of certain proteins to be made, and that causes problems at the cellular
level. For example, albinism is caused by a lack of an enzyme called Tyrosinase.
That missing enzyme means that the body cannot manufacture melanin, the
natural pigment that causes hair color, eye color and tanning. Because
of the lack of melanin, people with this genetic problem are extremely
sensitive to the UV rays in sunlight.
Cancer. Occasionally a cell will change in
a way that causes it to reproduce uncontrollably. For example, when cells
in the skin called melanocytes are damaged by ultraviolet radiation in
sunlight they change in a characteristic way into a cancerous form of cell.
The visible cancer that appears as a tumor on the skin is called melanoma.
(See the HSW
article on sunburn for more information)
Viral or bacterial infection. When a virus
or bacteria (also known generically as a germ) invades your body and reproduces,
it normally causes problems. Generally the germ's presence produces some
side effect that makes you sick. For example, the strep throat bacteria
(Streptococcus) releases a toxin that causes inflammation in your throat.
The polio virus releases toxins that destroy nerve cells (often leading
to paralysis). Some bacteria are benign or beneficial (for example, we
all have millions of bacteria in our intestines and they help digest food),
but many are harmful once they get into the body or the bloodstream.
The immune system also has several other important
jobs. For example, your immune system can detect cancer in early stages
and eliminate it in many cases.
It creates a barrier that prevents bacteria
and viruses from entering your body.
If a bacteria or virus does get into the body,
the immune system tries to detect and eliminate it before it can make itself
at home and reproduce.
If the virus or bacteria is able to reproduce
and start causing problems, your immune system is in charge of eliminating
Bacteria and Viruses
Your 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. Bateria 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.
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
Let's look at each of these components in
White blood cells
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.
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).
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
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 (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:
Whenever you see an abbreviation like IgE
in a medical document, you now know that what they are talking about is
Immunoglobulin A (IgA)
Immunoglobulin D (IgD)
Immunoglobulin E (IgE)
Immunoglobulin G (IgG)
Immunoglobulin M (IgM)
For additional information on antibodies
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,
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 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.
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:
Natural killer cells
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:
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 this
page to learn more about marrow transplants.)
Granulocytes - 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
Lymphocyte -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).
Monocytes - 7% or so of all leukocytes. Monocytes
evolve into macrophages.
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.
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.
Neutrophils - 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
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.
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 this
one 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.
article and this
article for additional details.
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."
all of this new 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.
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
How AIDS works
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. This
page shows the reproductive process of HIV very clearly.
There is a gigantic amount of AIDS literature
available on the web. These lists act as good starting points if you want
to learn more:
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.
When the Immune System
makes a Mistake
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.
None of this really matters, of course,
unless you get sick. That's when you notice that your immune system has
missed something. If you are sick now, I hope you get well soon. Whether
you are sick or not, I hope this article has helped you to understand and
gain a huge appreciation for the amazing system that keeps you well most
of the time - your immune system.