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Spelt is so close to normal bread wheat (closer than durum wheat, used for
pasta, is to normal bread wheat) that I would guess the chances of someone
being allergic to wheat and not to spelt as being something like 1 in a
million. By allergy, I shall arbitrarily limit my discussion to people who
primarily have respiratory symptoms or who develop skin wheals upon eating
wheat (these are associated with IgE type allergies) as opposed to people
who have gastrointestinal problems, which might result from either celiac
disease (especially involving IgA) or allergy.  Allergy may be caused by
proteins in wheat that are different from gluten proteins--at least this is
true for baker's asthma.  I have the definite impression that allergy may be
quite variable in any given individual--a person may react strongly one day
and weakly another, hence sometimes giving the impression that he/she can
handle spelt; psychological factors may also be involved.  On the other
hand, there is almost no question now that spelt is harmful to celiac
patients.  Spelt should be avoided by celiac patients.

Similarity of wheat, rye, and barley proteins:  Proteins are polymers of
amino acids.  The amino acids are joined together by peptide bonds like
beads on a string.  There are 20 different amino acids commonly found in
proteins and the sequence in which these 20 different amino acids appear in
the string is important to determining the type of protein.  Wheat, rye, and
barley all have proteins that differ by species; the proteins of barley can
be distinguished from the proteins of wheat, but wheat, rye, and barley
proteins have some considerable similarities in  amino acid sequence.
Although a typical gluten protein may have 300 amino acids in its chain, it
has been essentially proved that a 19-amino acid piece of that chain or
string can cause damage in celiac disease. Even smaller pieces may be
active. Some of the key suspect sequences appear in the wheat, barley, or
rye storage proteins. Traditionally, gluten (highly cohesive and elastic) is
found only in wheat and actually is made up of storage proteins, which
provide a source of nitrogen and amino acids for the new plant upon
germination of the seed.  A gluten ball is difficult or impossible to wash
from rye or barley, but celiac patients have come to call all toxic proteins
or peptides (pieces of the protein, usually resulting from the digestive
process) "gluten".  So when a celiac asks does a grain have gluten? I
translate in my head immediately into, "Does the grain have any proteins
that include any of the suspect sequences."  Strictly speaking, rye and
barley have storage proteins with some important similarities in sequence to
the wheat proteins.   The situation with oats is more complex.  Most of the
proteins do not have similarities, but a small fraction does have some
suspect sequences.  If oats holds up as toxic, then it is likely to be
because of these particular sequences.  No other plants are known to have
these key sequences, but, of course, the number of proteins sequenced from
various species is not all that large in comparison with the enormous number
of possiblities.  The exorphins and the celiac-active peptides appear
generally to consist of pieces having different amino acid sequences.

Donald D. Kasarda