Unbeknownst to most, soy is one of the most common food allergies. Found worldwide, this allergy typically appears only in infants, although it does manifest, more rarely, in children and adults. There are at least 15 soybean proteins that are allergens, but it is not clear whether soy contains one specific component that can be tagged as the main allergen.

Symptoms can include acne, angioedema, rhinitis, anaphylaxis, asthma, atopic dermatitis, bronchospasm, cankers, colitis, conjunctivitis, diarrhea, diffuse small bowel disease, dyspnea, eczema, enterocolitis, fever, hypotension, itching, laryngeal edema, lethargy, pollinosis, urticaria, vomiting, wheezing.

The intensity of the allergy varies from person to person, both from strength of reaction to what causes the reaction. The differing ways of preparing soy can remove some of or eliminate all the allergenic proteins in the beans. This means that Sally might be able to eat fermented soy products like natto or miso with impunity, but become ill from raw ones like edamame, while Jack has a reaction to anything containing soy at all, even soy bean oil, which contains no soy proteins at all.

See sensei’s node How Many Ways Can You Say "It's Stuff Made From Soy"? for a list of some products that contain soybeans.