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How allergic reactions work
An allergic reaction involves two features of the human immune response.
One is the production of immunoglobulin E (IgE), a type of protein called
an antibody that circulates through the blood. The other is the mast cell,
a specific cell that occurs in all body tissues but is especially common in
areas of the body that are typical sites of allergic reactions, including
the nose and throat, lungs, skin, and gastrointestinal tract. Food allergies
are usually inherited - if allergies of any kind are common in a family, then
developing food allergies will be more likely, especially if both parents
have allergies of some kind.
Before an allergic reaction can occur, the person has to be exposed to the
foods s/he is allergic to. As this food is digested, it triggers the allergic
response, generating these antibodies in large amounts which then attach to
these mast cells. The next time the person eats that food, it interacts with
those antibodies and triggers the mast cells to release chemicals such as
histamine. The location of the body these cells are in will determine the
location of the allergic response. For example, if these cells are in the
intestines, the person may have abdominal pain or diarrhea. The chemicals
released by mast cells in the skin can cause hives.
The digestion process affects the timing and the location of a reaction.
The initial reaction may be a tingling or itching sensation as they start
to eat the food, then reactions may occur in the stomach and abdominal region,
and as the allergens travel through the bloodstream, possibly a drop in blood
pressure, hives or eczema may occur as well, or even trigger an asthma attack
when the allergens reach the lungs. All or part of these types of reactions
can occur between near-instantaneous to an hour or so after ingesting the
offending food.
Common Allergens
Although an individual could be allergic to any food, such as fruits, vegetables,
and meats, they are not as common as the following eight foods which account
for 90% of all food-allergic reactions: Milk, Egg, Peanuts (one of the chief
foods that cause severe anaphylaxis), Tree nut (walnut, cashew, etc.), Fish,
Shellfish (shrimp, crayfish, lobster, crab and so on), Soy and Wheat (gluten).
Other allergens include medications, insect stings (such as bees), and latex
(particularly common among healthcare workers.)
Food allergies vary in severity, from life threatening to just a nuisance
rash. Reactions are sometimes dose-dependent: trace or small amounts may be
fine, but larger amounts may trigger a reaction, however severe. If you are
allergic to one specific food in a food family other foods in the same food
family (i.e., dairy products) may cause a similar reaction, even if you're
not already aware of it. The foods that adults or children react to are usually
those foods they eat more often. In Japan, for example, rice allergy is more
frequent. In Scandinavia, codfish allergy is more common.
Cross Reactivity
If someone has a life-threatening reaction to a certain food, doctors will
usually advise them to avoid similar foods. For example, a history of allergy
to shrimp will usually indicate cross-reactivity to other shellfish such as
lobster or crab as well.
During pollen seasons, people allergic to particular pollens or ragweed may
find they have a reaction when eating melons or fruits. This is likely due
to pollen dust resting on the skin of the fruit, or when the fruit is sliced
(such as cantaloupe), some of the pollen dust then gets dragged into the rest
of the fruit on the surface of the knife.
Anaphylaxis
A sudden, severe, potentially fatal, systemic allergic reaction that can
involve various areas of the body (such as the skin, respiratory tract, gastrointestinal
tract, and cardiovascular system). Symptoms occur within minutes to two hours
after contact with the allergy-causing substance, but in rare instances may
occur up to four hours later. Anaphylactic reactions can be mild to life-threatening.
Common causes of anaphylaxis include: Food (such as peanuts), medications,
insect stings (bee stings being a common severe allergen), and latex (particularly
common among healthcare workers). Anyone with a history of anaphylactic reactions
is at risk for another severe reaction.
Differential Diagnoses
While "allergy" and "intolerance" are often used interchangeably,
they are really two different things. A food intolerance is an adverse food-induced
reaction that does NOT involve the immune system - lactose intolerance is
one example, or MSG sensitivity, or reactions to things like Nutrasweet. When
a lactose intolerant person eats or drinks something that is milk based, symptoms
such as gas, bloating, or abdominal pain may occur because the person's body
lacks an enzyme that is needed to digest milk products. This usually does
not always have the potential to be fatal, merely inconvenient, painful or
embarrassing.
Food poisoning:
Sometimes food poisoning symptoms will mimic the symptoms of a food allergy.
Adverse Reactions:
Another intolerance example is an adverse reaction to certain components
that are included in food to enhance taste, color, or preserve it. Such
substances can include artificial colors and flavors (One reaction to artificial
colors/flavors in foods, particularly in children, is "hyperactivity"
or attention deficit disorder. It's not, however, the only ADD cause.) MSG
and sulfites are other items that can generate an intolerant reaction.
Gluten sensitivity, or Celiac Syndrome, is another common adverse reaction
- this is cause by an abnormal immune response to gluten, which is a component
in wheat and other grains. Because gluten and gluten byproducts make their
way into a LOT of foods, someone with this problem has to spend a lot of time
reading labels to be sure there's not wheat or gluten products.
Discovering Food Allergies
The first step is to find out what you're allergic to. Keep a food diary
for at least 1-2 weeks. Log any and all negative reactions you get, regardless
of when they happen. Log any and all foods you eat, drink or snack on, even
if it's just "a nibble of so-and-so's sandwich". Log anything you
eat or drink in-between meals as well. Log all medication you may be taking
(sometimes a medication taken on an empty stomach will give you abdominal
pains, but be OK once you've had food) and when you take it. This information
can tell you (or your doctor) what you may be allergic to.
Questions to keep in mind when keeping a food diary (and your doctor will
most likely ask similar questions, if not more) are:
What was the timing of the reaction? Did the reaction come on quickly,
usually within an hour after eating the food?
Is the reaction always associated with a certain food?
Did anyone else get sick? For example, if the person has eaten fish contaminated
with histamine, everyone who ate the fish should be sick. In an allergic
reaction, however, only the person
allergic to the fish becomes ill.
How much did the patient eat before experiencing a reaction? The severity
of the patient’s reaction is sometimes related to the amount of food
the patient ate.
How was the food prepared? Some people will have a violent allergic reaction
only to raw or undercooked fish. Complete cooking of the fish destroys those
allergens in the fish to which they react. If the fish is cooked thoroughly,
they can eat it with no allergic reaction.
Were other foods ingested at the same time of the allergic reaction? Some
foods may delay digestion and thus delay the onset of the allergic reaction.
The next step is a true elimination diet. Pick a food or food group (i.e.,
milk or dairy products) and eliminate it for two weeks to a month. Observe
your reactions, whether they change or stay the same. After this period of
time, put that food/food group back in your diet and eliminate another food/food
group (i.e., shellfish) for another two weeks to a month. Again, observe your
reactions, whether they change or stay the same. Repeat this until you've
worked through all the foods you usually eat, or you find out what causes
a reaction, whichever comes first.
Once you've gone through the elimination diet, you should know what foods
you're allergic to, or if you're allergic to any foods. (Sometimes what can
seem like a food allergy can be a case of food poisoning due to bad food preparation/storage
in a restaurant.)
Treating food allergies
Food allergies are easily treated by dietary avoidance - once the food allergens
are identified, the food must be removed from the person's diet. This requires
reading of lengthy ingredient lists for each item they are considering eating.
Foods such as peanuts, eggs and milk often show up in foods that would not
normally be associated with them, or there are trace amounts stemming from
the equipment used in the preparing factory.
People that are highly allergic to an item (such as peanuts) cannot tolerate
even the smallest amounts without having a reaction. People who's allergies
are mild to moderate can tolerate small amounts with little to no discomfort.
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