Soy milk is the most allergenic common soy product, and it triggers eczema through two completely different pathways depending on the individual. The first pathway affects people with birch pollen allergy — soy milk contains a protein that looks almost identical to birch pollen to the immune system, so the body attacks it. This protein is destroyed by heat, which is why these people usually tolerate cooked soy products like tofu and soy sauce but react specifically to soy milk.

The second pathway involves heat-stable storage proteins that survive all forms of cooking — people with this pattern react to all soy products regardless of preparation. Soy is one of the top eight allergens recommended for investigation in children under 5 with moderate-to-severe eczema.