Last reviewed: February 15, 2025

Last updated: February 15, 2025

Written by: Jay Hastings, CEO of PlexusDx

Jay Hastings is the CEO of PlexusDx, a precision health company focused on genetic testing, blood biomarker insights, and personalized wellness recommendations. He has more than 20 years of experience across healthcare innovation, genomics, laboratory operations, healthcare investing, and strategic finance.

Medically reviewed by: Jayden Lee, PharmD, EMBA

Jayden Lee, PharmD, EMBA, is the PlexusDx Medical Science Liaison with a PharmD and MBA specializing in pharmacogenomics and clinical product development, with a proven ability to bridge the gap between genomic research and practical patient outcomes. Dr. Lee has more than 10 years of professional experience in clinical pharmacy, academia, and research.

The difference is which system in your body is reacting. A food allergy is an immune response — according to the FDA, food allergies occur when the body's immune system reacts to certain proteins in food, and reactions range from hives and lip swelling to anaphylaxis, which can be fatal. A food sensitivity or intolerance is usually a digestive or metabolic problem — the classic example being insufficient lactase to digest milk sugar — and it does not involve the same immune machinery or carry the same risk of a rapid, life-threatening reaction.

What Happens in a True Food Allergy

The immune system misidentifies a food protein as a threat and mounts a response against it. In the classic immediate form, that response is mediated by immunoglobulin E antibodies, and symptoms typically appear within minutes to about two hours.

Symptoms can involve skin, gastrointestinal tract, respiratory tract and cardiovascular system, often simultaneously. The FDA describes severity ranging from mild symptoms involving hives and lip swelling to severe, life-threatening symptoms often called anaphylaxis, which may involve fatal respiratory problems and shock.

Crucially, the amount of food required can be very small. This is why allergen labeling law exists at all, and why cross-contact during manufacturing is treated as a serious hazard rather than a rounding error.

What a Food Sensitivity Usually Is

Most food intolerances are digestive rather than immune. Lactose intolerance is the canonical case: insufficient lactase activity leaves milk sugar undigested, colonic bacteria ferment it, and the result is bloating, flatulence, abdominal pain and diarrhea, typically 30 minutes to 2 hours after eating.

Others are pharmacological or chemical — reactions to caffeine, to histamine in aged foods, or to certain additives. Some are dose-dependent in a way allergies are not: a small amount may cause nothing while a larger amount causes symptoms.

The practical hallmark is that symptoms are usually confined to the digestive tract and are uncomfortable rather than dangerous. That is a general pattern, not a guarantee, and it is not a substitute for evaluation.

The Category That Belongs to Neither

Celiac disease is an autoimmune condition triggered by gluten, and it is neither an allergy nor an intolerance. The National Institute of Diabetes and Digestive and Kidney Diseases describes it as a condition in which eating gluten causes damage to the small intestine.

That distinction matters because the consequences of continued exposure differ. Celiac disease involves ongoing intestinal damage with downstream nutritional consequences, which is a different proposition from transient digestive discomfort.

Diagnosis also differs. Celiac disease has established serologic and biopsy-based diagnostic pathways, and — importantly — testing is generally performed while gluten is still in the diet. Eliminating gluten before testing can make an accurate diagnosis considerably harder.

The Nine Major Food Allergens

The Food Allergen Labeling and Consumer Protection Act of 2004 identified eight foods as major food allergens: milk, eggs, fish, Crustacean shellfish, tree nuts, peanuts, wheat and soybeans.

The Food Allergy Safety, Treatment, Education, and Research Act, signed into law on April 23, 2021, declared sesame the ninth major food allergen recognised in the United States, effective January 1, 2023.

Those nine drive the labeling requirements you see on packaged food. The FDA has also issued guidance outlining how it evaluates the public health importance of food allergens other than the nine identified by law.

Why Testing Is Where People Go Wrong

Allergy evaluation is clinical. Skin prick testing and specific IgE blood testing are interpreted alongside a detailed history, because a positive result without a matching clinical history often reflects sensitisation rather than clinical allergy. Oral food challenge remains the reference standard and is performed under medical supervision.

Direct-to-consumer panels measuring food-specific IgG are a different matter. IgG antibodies to foods are generally understood to reflect exposure, and major allergy organisations have repeatedly cautioned against using them to diagnose food allergy or to drive elimination diets.

The cost of over-testing is real: unnecessary elimination diets narrow nutrition, create anxiety around eating, and can delay identification of a treatable condition such as celiac disease or inflammatory bowel disease.

What to Do With Symptoms You Cannot Explain

Keep a structured record — what was eaten, how much, how long until symptoms, what the symptoms were, and how long they lasted. Timing and reproducibility are the most useful clues a clinician can work with.

Seek urgent evaluation for anything involving breathing difficulty, throat tightness, widespread hives, faintness or swelling of the lips, tongue or face. Those features point toward allergy rather than intolerance and are time-critical.

For persistent digestive symptoms without those features, start with a clinician rather than an elimination diet. MedlinePlus and the FDA both emphasise that accurate identification is what makes management work, and food allergies currently cannot be cured — early recognition and learning which foods to avoid are the measures that prevent serious consequences.

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Frequently Asked Questions

What is the main difference between a food allergy and a food sensitivity?

Which system reacts. A food allergy is an immune response to a food protein and, per the FDA, can range from hives and lip swelling to anaphylaxis with fatal respiratory problems and shock. A food sensitivity or intolerance is usually digestive or metabolic — such as insufficient lactase to digest milk sugar — producing discomfort rather than a life-threatening reaction.

Which foods are the major allergens in the United States?

Nine. The Food Allergen Labeling and Consumer Protection Act of 2004 identified milk, eggs, fish, Crustacean shellfish, tree nuts, peanuts, wheat and soybeans. The FASTER Act, signed April 23, 2021, declared sesame the ninth major food allergen, effective January 1, 2023. These drive the allergen labeling requirements on packaged foods.

Is celiac disease a food allergy?

No. Celiac disease is an autoimmune condition in which eating gluten causes damage to the small intestine, as described by the National Institute of Diabetes and Digestive and Kidney Diseases. It is neither an allergy nor a simple intolerance, it has established serologic and biopsy-based diagnostic pathways, and testing is generally done while gluten is still in the diet.

Are food sensitivity blood tests reliable?

Direct-to-consumer panels measuring food-specific IgG are widely criticised, because IgG antibodies to foods are generally understood to reflect exposure rather than disease, and major allergy organisations have cautioned against using them to diagnose food allergy or drive elimination diets. Allergy evaluation relies on history plus skin prick or specific IgE testing, interpreted clinically.

When should symptoms be treated as an emergency?

Any reaction involving breathing difficulty, throat tightness, widespread hives, faintness, or swelling of the lips, tongue or face requires urgent medical attention. Those features point toward an allergic reaction rather than an intolerance, and the FDA notes anaphylaxis may involve fatal respiratory problems and shock. Digestive discomfort alone warrants clinical evaluation, not emergency care.

Medical and Editorial Standards

Medical review process: This article was reviewed for medical accuracy, scientific clarity, evidence alignment, and appropriate discussion of genetics, medications, supplements, biomarkers, and health-related claims.

Sources and evidence: PlexusDx educational content is developed using peer-reviewed research, clinical literature, reputable medical references, and, where applicable, public health or regulatory guidance.

Commercial transparency: PlexusDx offers genetic testing, blood biomarker testing, personalized supplement recommendations, and related precision wellness services. Product mentions are intended to help readers understand available options and should not be interpreted as medical advice.

Important disclaimer: PlexusDx educational content is for informational purposes only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about medications, supplements, genetic testing, lab testing, or health-related care.