Most of the time, a soy allergy is more annoying and uncomfortable than it is serious. People who have soy allergies develop a negative-but usually mild-reaction after ingesting soy. Often, these reactions include rashes, swelling, wheezing, and nausea. However, soy allergies can occasionally be serious and even fatal. People who are strongly allergic to soy can experience anaphylactic shock, which consists of severe airway constriction, loss of consciousness, and a rapid but weak pulse.