Alex,

The following information is from _Food Allergies and Food Intolerance: The
Complete Guide to Their Identification and Treatment_ by Jonathan Brostoff,
MD and Linda Gamlin, pp. 25-33.  I've condensed the info a lot, so for those
of you who want to know more I highly recommend this book.  I just started
reading it recently and it has invaluable information.  This is what I get
so far:

IgE is a type (isotype) of an antibody produced by the body to help fight
off infection.  Antibodies actually bind to antigens (protein or
carbohydrate substances that trigger antibodies) and act as markers to
identify invaders (virus, bacteria, etc.) to other cells in the body who
will then fight the invasion.  The different isotypes of antibody are IgG,
IgA, IgM, IgD, and IgE.  Ig just stands for Immunoglobulin, another name for
antibody.  The main function of IgE is to defend the body against infections
larger than bacteria and viruses (e.g., ringworms,flukes).

IgE molecules are different from other antibodies in that they attach
themselves to mast cells (a type of cell in solid body tissue) and then wait
for antigens.  Once the antigens have bound to the IgE antibody, the mast
cell that's on the other end releases chemicals called mediators.  Mediators
cause blood vessels to open or cause cells to contract or become leaky or
any number of other reactions that lead to physical symptoms such as
diarrhea, rash, behavior problems, asthma, etc.  Histamine is one of these
mediators.

In nonallergic people, the level of IgE in the bloodstream is usually very
low.  In allergic people, for some reason, this process has gone wrong and
the IgE molecules decide to attack something that isn't attacked in
nonallergic people: e.g., milk protein or peanut protein or cat dander
(protein), etc.

Consequently, allergic people tend to have higher levels of IgE in their
bloodstream.  But this is not always true.  Some have normal levels.  And
some people have high IgE and no allergic symptoms.  The picture is more
complex than this, but this is one piece of the puzzle.

Cheryl