More from Phil in response to another question (part of the question is
after his reply since our system does not allow us to cut and paste.

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   I am no expert on allergies, so I cannot really say if it is the
sulfur in the sulfonamides or the actual shape of the molecules that
cause the allergy.  However, keep in mind that sulfur is a necessary
element for life and many vital molecules ranging from vitamins to
amino acids contain sulfur.   Remember the mnemonic taught in some
schools to help remember the necessary elements in proteins:
S.P. COHN (Sulfur, Phosphorus, Carbon, Oxygen, Hydrogen, Nitrogen).
  Most allergens, even the really bad ones, contain the same elements
that are in harmless compounds.  For example, urushiol, the main
allergen in poison ivy contains just carbon, oxygen and hydrogen, and
if you take a very quick glance at its structure, it looks a little
like vitamin A.  So its the shape of the molecule that matters most.
  Whether people who are allergic to sulfa drugs are also allergic to
the sulfur compounds in cabbage is a good question.  Is an allergy to
sulfa drugs an indication that that person may be more allergy prone
than other people?  Only an allergist would know.

The above is in answer to this question:
Yes, but Phil, do you think it is the sulfur in sulfonamides that is
responsible for allergies to this class of drugs, or the unique
molecular composition?  The original question of this thread had to do
with whether or not people with allergies to sulfa drugs could be
likewise allergic to cabbage compresses.