An
allergen
is
any
substance
that
causes
an
allergic
reaction,
most
often
when
eaten
or
inhaled.
No
comprehensive
list
of
allergens
is
possible,
because
sensitivities
vary
from
one
person
to
another
and
it
is
possible
to
be
allergic
to
literally
anything.
Dust,
pollen
and
pet
dander
or
pet
hair
are
all
common
allergens,
but
it
is
possible
to
be
allergic
to
anything
from
chlorine
to
perfume.
Food
allergies
are
not
as
common
as
food
sensitivity,
but
some
foods
such
as
peanuts
(really
a
legume),
nutss,
seafood
and
shellfish
are
the
cause
of
serious
allergies
in
many
people.
A
few
people
have
even
been
recorded
to
be
allergic
to
certain
chemicals
found
in
almost
all
water.
Poison
ivy
(and
other
plants,
like
poison
sumac
or
poison
oak)
is
a
plant
that
will
cause
an
allergic
reaction
for
anyone,
given
enough
repeated
contact
--
like
any
allergy,
the
human
body
must
learn
to
fight
the
allergen,
some
bodies
learn
slower
and
will
appear
to
be
"immune"
to
poison
ivy.
An
allergic
reaction
can
be
caused
by
any
form
of
direct
contact
with
the
allergen
-
eating
or
drinking
a
food
you
are
sensitive
to
(ingestion),
breathing
in
pollen,
perfume
or
pet
dander
(inhalation),
or
brushing
your
body
against
an
allergy-causing
plant
(direct
contact,
generally
resulting
in
hives).
Other
common
causes
of
serious
allergy
are
bee
stings,
penicillin,
and
latex.
An
extremely
serious
form
of
an
allergic
reaction,
which
can
kill
in
mere
minutes,
is
called
anaphylaxis.