Soy allergy is an immune system response to proteins that are in soy. At least 16 potential soy protein allergens have been identified but their relative clinical significance is unknown.

About 0.5% of the population has soy allergy, but it is more common in infants: the average age at which the allergy manifests is 3 months but the majority of infants outgrow it by the age of two. Adults with soy allergy are rare.

Soy protein is also a common allergen; the reported soy allergy symptoms include: acne, angioedema, rhinitis, anaphylaxis, asthma, atopic dermatitis, bronchospasm, colitis, conjunctivitis, diarrhea, dyspnea, eczema, enterocolitis, fever, hypotension, itching, laryngeal edema, lethargy, hives, vomiting and wheezing.

If soy allergy is suspected, soy should be completely removed from the diet for several days. Soy products can then be cautiously returned to the diet to see if symptoms return. Intolerance reactions can occur as long as 48 hours after ingestion of an offending substance.

Soy allergy symptoms are similar to Cow Milk Allergy symptoms and occur within few hours after soy ingestion and usually disappear after elimination of soy from the diet. Several studies show that, among infants with Cow Milk Allergy, those who were fed soy protein based formulas, some 30% to 50% concurrently suffered from soy protein allergy.

The way in which soy is processed can affect allergenicity and many people allergic to soy may also react to other foods, especially other legumes such as peas, peanuts, lentils and garbanzo beans.