[1] It is a hypersensitivity to ingesting compounds in soy (Glycine max), causing an overreaction of the immune system, typically with physical symptoms, such as gastrointestinal discomfort, respiratory distress, or a skin reaction.

[1] The most severe food allergy reaction is anaphylaxis,[1] which is a medical emergency requiring immediate attention and treatment with epinephrine.

[4] Symptoms may include: rash, hives, itching of the mouth, lips, tongue, throat, eyes, skin, or other areas, swelling of lips, tongue, eyelids, or the whole face, difficulty swallowing, runny or congested nose, hoarse voice, wheezing, shortness of breath, diarrhea, abdominal pain, lightheadedness, fainting, nausea and vomiting.

[1][4] Anaphylaxis occurs when IgE antibodies are released into the systemic circulation in response to the allergen, affecting multiple organs with severe symptoms.

FPIES can be severe, even leading to anaphylaxis characterized by persistent vomiting 1–4 hours after an allergen-containing food, to the point of lethargy.

[6][11][12] Watery and sometimes bloody diarrhea can develop 5–10 hours after the triggering meal, to the point of dehydration and low blood pressure.

[13] Allergic reactions are hyperactive responses of the immune system to substances that are normal components of foods, such as proteins.

[1] Conditions caused by food allergies are classified into three groups according to the mechanism of the allergic response:[14] In the early stages of an acute IgE-mediated allergic reaction, lymphocytes previously sensitized to a specific protein or protein fraction react by quickly producing a particular type of antibody known as secreted IgE (sIgE), which circulates in the blood and binds to IgE-specific receptors on the surface of other kinds of immune cells called mast cells and basophils.

[15] Activated mast cells and basophils undergo a process called degranulation, during which they release histamine and other inflammatory chemical mediators into the surrounding tissue causing effects, such as vasodilation, mucous secretion, nerve stimulation, and smooth-muscle contraction.

[4] Depending on the individual, the allergen, and the mode of introduction, the symptoms can be system-wide (general anaphylaxis), or localized to the respiratory system (asthma) or skin (eczema).

[4][15] After the chemical mediators of the acute response subside, typically 2–24 hours after the original reaction, late-phase responses referred to as non-IgE mediated can occur due to the migration of other types of white blood cells to the initial reaction sites, causing diarrhea and other signs of gastrointestinal upset.

[22][23] Treatment for accidental ingestion of soy products by allergic individuals varies depending on the sensitivity of the person.

[33] Because it is only slightly soluble in water, it is incorporated into a lipid emulsion containing 10% refined soybean oil and 1.2% highly purified egg lecithin.

[37] The Nebraska government states that soy proteins from processed foods in the breast milk of nursing mothers may cause intolerance in infants.

[37] In opposition, one review stated that there was not yet sufficient clinical evidence to conclude that maternal dietary food avoidance during lactation would prevent or treat allergic symptoms from soy in breastfed infants.

[4] With awareness about the potential for food allergy increasing during the early 21st century, the quality of life for families and caregivers was affected.

[3] Despite these precautions, people with serious allergies are aware that accidental exposure can easily occur in family homes, school or restaurants.

Except in Canada and Brazil, there are no labeling laws to declare the presence of trace amounts in the final product as a consequence of cross-contamination.

[64] The US FDA requires reporting by government inspectors and manufacturers to reduce or eliminate the potential cross-contact of product ingredients, including soy, and to state possible cross-contamination on food labels, using the words "may contain".