Soy, a legume like its cousin the peanut, is a common allergen for infants. Luckily, most children outgrow the allergy by the age of 2, and soy allergies are rare in adults. Symptoms are as varied and include acne, nasal congestion, canker sores, conjunctivitis, fatigue, anaphylaxis and low blood pressure. Soy can be difficult to avoid, as it is found in processed meats, baked goods and many desserts.