Naomi,
A formula fed mom passing on allergies to her breastfed infant smacks of
Lamarkian evolution...as far as we know, we do not pass on aquired
characteristics.  With this caveat, I can think of a possible mechanism here:
 Human milk is not only nutrition, but also has trophic (growth and maturity
promoting) functions.  If mom's gi system never developed properly because she
was artificially fed, she might not correctly break down proteins, and foreign
dietary proteins could pass whole into her milk, and sensitize her breastfed
infant.  This of course is a very iffy scenario, as we know there is a strong
genetic susceptibility to allergies...
Catherine Watson Genna, IBCLC  who loves to play "what if"