~~~ What the Heck is an Allergy?
~~~
Allergy - Allergic - bummer! Get that pollen grain away from me! An
allergic reaction is a physiologic
response by our body against: something we have swallowed;
something we have inhaled;
something which has touched our skin; or, something injected into us by a
bite, a sting from an insect, or by a syringe needle (a
"shot" administered by medical personnel.) There are several
different kinds of allergies, e.g., immediate-type, delayed-type, Types I
- IV, etc., and against many different things,
e.g, food, pollen (see the above image), grass, chemicals, or metals. But,
the one with which most of us
are familiar is the allergic reaction that leads to immediate kinds of
responses such as sneezing, swollen eyes, runny nose, hives, itching, and
sometimes breathing
difficulties. The condition known as asthma is really an allergic
reaction against something which primarily affects the lungs; this one can
therefore cause serious breathing
difficulties - although others can as well to a greater or lesser
extent. In some cases we may become sick to our stomach, or break out in
a bright rash - there can be several different symptoms. And, in some
instances the allergic reaction can be so severe that as a result we may go
into shock (rapid loss of blood pressure - can lead to heart failure and
brain shut-down) and possibly die unless we quickly obtain medical
assistance. In fact, allergies are so common and cause so many medical
and personal problems, that there is an entire governmental research
institute devoted to the study of allergies. This institute is called
the National Institute of Allergy and Infectious Diseases, or,
NIAID. This particular institute
is only one among many within the National Institutes of Health (NIH)
that our tax dollars support and which help find cures for the many human
diseases that afflict us.
Allergic responses are linked to our immune
system - and specifically to antibodies - here
we go again, talking about antibodies, proteins produced by our immune
system which circulate in our bloodstream and help to remove any substance
(toxin) or organism (virus,
bacterium) which has invaded our body. In this particular instance, we
are going to talk about the immediate kind of allergic reaction that we
may experience and which involves a particular kind of antibody,
named IgE (immunoglobulin E). This class of antibody is one of the
five different classes of antibody made by our immune system (IgM, IgG,
IgD, IgA, and IgE).
Each antibody molecule is capable of binding to a distinct
three-dimensional molecular shape present on substances such as proteins
or complex sugars, whether or not the substance is floating around
freely, or whether the substance is an integral part of an
organism. Thus, whenever a substance or complete organism invades our
body, the immune system can respond to this intrusion by synthesis of
antibodies. If we happen to make IgE-class antibody against certain
things, we can be in trouble (mild or severe).
Here is why:
All antibody molecules are shaped like the letter "Y."
Each arm of the Y has a site which can bind to a given molecular shape,
and the bottom part (the leg of the Y) can also serve to generate certain
responses when
something else binds to IT. The image shows one of the two binding sites of an
antibody molecule (one of the two arms of the Y, called Fab - fragment,
antigen binding - the
heavy-chain in blue, light-chain in green, wrapping around one another
to form the site, and a protein antigen in red being held within the
site; the bottom part, the leg of the molecule, is not shown).
In the case of IgE, both the top and bottom
parts of the antibody molecule can lead to problems for us. The bottom
part of the
Y-shaped IgE molecule (called the Fc for fragment-crystallizing) can
interact with and be bound by a cell receptor
(a protein in the membrane which can "grab" and "hold
onto" a particular thing) specific for
this part of the IgE molecule (and nothing else). The receptor is
located in the membrane
of cells named mast cells and basophils. Mast cells are all over the
place within our skin, and basophils circulate within our bloodstream.
As IgE circulates, it may come in contact with mast cells or basophils,
and through interaction with the receptor can specifically adhere to the
membrane of these cells with the arms of the Y sticking out towards the
surrounding environment, and the leg part of the molecule embedded
within the membrane - would be analogous to you, standing with your
feet and ankles sunk into wet cement, and your arms raised above your
head. Now, if the substance against which the
IgE was originally made appears and bumps into a mast cell or basophil
within the skin or within the
bloodstream respectively, the binding sites at the end of the arms of the
IgE molecule may bind and hold onto the
substance. If there are two Y Y (IgE molecules) next to one another in
the membrane, and each of their "hands" simultaneously binds the
substance (generates a
cross-link), this interaction "wiggles" the membrane and triggers
the mast cell or
basophil (as the case may be) to release all kinds of powerful substances.
One of the substances released by the mast cell and/or basophil is called
histamine. This chemical can interact with other cell receptors specific
for histamine, and trigger these cells to release other powerful
substances as well. One of the things that happens when histamine is
released is that nearby tissue changes - the places where red blood cells
interact with tissue - called capillaries - increase in volume (called
vascular dilation). The result of this increase in volume means that
more blood can get to the site (is the reason why the spot will be warmer
and redder
than surrounding skin). Also, the amount of secretions increase (lymph
fluid) - is why the area swells (edema). This increase in blood-flow,
and lymph-flow to the site, can cause a raising of the skin which may
itch, and which becomes red (blood near the surface of the stretched,
raised, tissue) - is the rash (erythema). Now you know why we take
anti-histamines to relieve these symptoms, and other chemicals to dry up
our runny nose. The allergic reaction might be pretty
mild - although the rash may itch like all get out - these substances
must stimulate "itch" nerves... nahhh, just kidding - but -
nerves are indeed stimulated in a certain way - helps let us know
that something is going on. All in all though, a mild reaction can
simply be just a bother,
and not particularly threatening. On the other hand, if instead of the
skin these very same reactions occur within the lungs or within the
bloodstream, we can be in serious trouble.
When IgE sticking from mast cells in the lung tissue binds to an
invading (inhaled) substance like pollen grains,
all of those substances just mentioned are released. The resultant
increase in
lymph-secretion in the lungs can cause the lungs to fill with
fluid. Now, I know that you know that liquids cannot be compressed -
cannot be squeezed down into a smaller volume - no matter how much pressure
you place upon them - is why hydraulic fluid works - so, if the lungs
fill with fluid, our muscles are not strong enough to squeeze our lungs
properly, and we are then unable to breathe. Too, the allergic response
chemicals can lead to swelling of the tissue - decreasing the diameter of
our airway pipes (bronchial tubes). Also, the slow-reacting substance of
anaphylaxis (SRA) causes the breathing muscles to become constricted
which squeezes the bronchial tubes (airways),
making it yet more difficult to breathe. This condition would be an
asthmatic
condition - a scary, uncomfortable, and sometimes dangerous condition. If
the reactions occur within the
bloodstream via IgE bound to basophils, we can be in yet deeper trouble.
The chemicals released by the basophil will rapidly circulate all over the
body - there will be blood-flow and lymph-flow increases all over the place.
This rapid loss of blood into tissues via the capillary beds will result
in a rapid drop in blood pressure. The brain will not receive proper
oxygen flow; the heart will strain to function; the lungs will fill
with fluid; the airways will become severely restricted - breathing
muscles become severely constricted - these things
together are called anaphylactic shock. If a person does not receive
help, the person can die. It is for this reason that a person allergic
to bee/wasp venom can be at significant risk. If a bee or wasp stings a
person, the venom enters the bloodstream directly, and circulates within
minutes all over the body. The allergic reactions also occur within
minutes, and a person can be in serious difficulty in a very short time.
It is for this reason that if you are about to receive an injection of
any kind from your health care provider, that you are asked if you are
allergic to so-and-so. Examples are: receiving anesthetics of
any kind (prior to dental work or surgery); receiving injections of or
swallowing antibiotics (penicillin); receiving a flu shot (egg protein
- the virus is grown in chicken eggs); receiving any vaccination.
Sometimes a person will not know whether or not they are allergic to
something - it is for this reason that after you receive a vaccination
that you are asked to wait around in the physician's office for
awhile - just in case. If you are at the dentist or are about to undergo
surgery, you'll have to wait, anyway.....
How do we prevent these reactions - or at least - how do we overcome an
allergic reaction? The easiest way to prevent allergies is to avoid
coming into contact with the substance against which you are allergic
(the allergen). In some instances, avoiding exposure is difficult. As
it turns out, we have natural responses of our own, some
pretty neat physiologic things
that happen automatically - but sometimes our automatic responses are
overwhelmed and we
need outside assistance. Let's say that you are experiencing an asthma
attack. You will begin to have trouble breathing - this result will
frighten you - the fright you feel causes the brain to send
"fright-response" substances out and about, which bind to
specific receptors for these things on the cells of
the adrenal glands located on your kidneys. The adrenal gland cells in turn
release the substance adrenalin (epinephrine) which has several powerful
effects. One important effect is to stimulate an enzyme to breakdown
glycogen - a glucose-sugar storage polymer in liver and muscles - into
individual glucose
molecules. These glucose molecules are rapidly "burned" by
cells and the energy released speeds-up all kinds of metabolic activity -
including the destruction of already-released histamine and the other
allergic-response substances - this response is known as the fight and
flight response - your muscles may even quiver with
all of the energy available - your strength will increase - sometimes
astonishingly - allows you to run like heck to get away from danger.
The heart will beat faster, and stronger too. Also, adrenalin begins almost
immediately to reverse
the effects of histamine, by causing basophils and mast cells to stop
releasing this chemical. Consequently, the stopping of histamine
release and the increased metabolic activity which destroys histamine
quicker than normal, both result in the lowering
of the amount of histamine throughout the body. This lowering of the
amounts of allergic-response chemicals, results in decreased tissue
swelling, decreased fluid accumulation, etc - and blood-pressure rises,
breathing returns to normal, and rashes and the like gradually go away.
Now, sometimes there just isn't enough adrenalin produced in time to
reverse the reactions quickly enough - in this instance one may need
outside
assistance by receiving an injection of adrenalin along with other
substances in liquid, tablet, or injection form which cause swelling to go
down, lung-associated muscle constriction to ease, airways to open, and
fluid accumulation to stop. People who are severely allergic to bee/wasp
venom usually carry around a little syringe filled with adrenalin when
they are in the woods, camping, or somewhere far from a medical
facility or phone.
Why do we even make IgE in the first place if all we seem to get is
trouble? Well, actually, we aren't sure - but- there is pretty strong
evidence that IgE is really good at protecting us from parasitic
infections. Those darn scallywag mast cells and basophils, if connected
to a parasite via the IgE bridges (become the "hands" of the
basophil and mast cells) release a bunch of things - and 'though these
things if released around and about within our body cause us trouble, these
same things are
deadly to a parasite. So apparently like everything else in life, under
some circumstances actions are helpful, and under different circumstances
these very same actions can get us into trouble. Apparently, the trick is
to be able to recognize the circumstance!
Book: Don't Touch That Doorknob!