Food allergies occur when the immune system overreacts to specific components of a food, resulting in such symptoms as skin hives or rashes, swelling, and trouble breathing. Soy allergy is a condition where a person is allergic to proteins naturally found in soy, but soy lecithin allergy is a separate allergy. Lecithin is a byproduct of soy processing, and is made up of fatty substances that are useful as emulsifiers in the food industry. People who have a soy allergy may not display allergic symptoms when they eat soy lecithin, as less allergens tend to be present.

Soybeans are common ingredients in Asian cooking. As well as being edible whole, they are also commonly processed into several different forms. Major forms include soy flour, tofu and soybean oil. Lecithin is a byproduct of the oil extraction process, and it can confer various desirable characteristics to processed food such as stabilization and emulsification, although it is not a food product itself.

Soy has a relatively high protein content, compared to many other vegetables. It is the proteins in soy that can adversely affect the human immune system. When people with soy allergy or soy lecithin allergy eat soy products, the immune system wrongly identifies the proteins as dangerous to health, triggering a potentially adverse reaction. The immune system sets off a strong inflammation process, which produces the swelling, redness and hives that are characteristic of allergy.

People with an allergy to soy typically have to avoid all soy foods, but products with soy lecithin may not pose a problem, as not as much protein is present. Those who do have allergic reactions to the lecithin suffer from soy lecithin allergy, which is a distinct condition from soy allergy and can be much harder to avoid. Many regular products contain soy lecithin as a minor ingredient, which may not be obvious from the type of food.

Often, those with soy lecithin allergy also have allergies to other substances because the immune system recognizes similar proteins present in related foods or non-related substances. An example of an allergy that may be present alongside allergies to soy components is birch pollen allergy. Common allergies such as those to nuts do not appear to be linked to soy allergies. People who suffer from soy lecithin allergy should also be aware that the lecithin is not only present in foodstuffs, but can also be present in cosmetics and medications.