Casein and Whey proteins are the two major allergenic proteins found in milk.
Overview of Milk Allergy
Milk allergy is a common food allergy, particularly in infants and young children, but it can persist into adulthood. It is an immune response to milk proteins, primarily casein and whey.
Milk Sources That Can Trigger Allergic Reactions
- Cow’s Milk (Most common)
- Goat’s Milk
- Sheep’s Milk
- Buffalo Milk
- Other Mammalian Milk Sources (e.g., camel, horse)
- Labeling note: under FDA guidance (January 2025), milk from goats, sheep and other domesticated ruminants counts as the major allergen “milk” and must be declared, with the animal named.
Allergenic Proteins in Milk
| Allergen | Protein | Molecular weight (kDa) | Stability | Cross-reactivity |
|---|---|---|---|---|
| Bos d 4 | Alpha-lactalbumin (whey) | 14.2 | Partly heat-labile | Goat, sheep and buffalo milk |
| Bos d 5 | Beta-lactoglobulin (whey; lipocalin family; absent from human milk) | 18.3 | Partly heat-labile; changed by extensive heating | Goat, sheep and buffalo milk |
| Bos d 6 | Serum albumin (whey) | 67 | Heat-labile | Raw or undercooked beef |
| Bos d 7 | Immunoglobulin (whey) | 160 | Heat-labile | — |
| Bos d 8 | Caseins: Bos d 9 (alpha-s1), Bos d 10 (alpha-s2), Bos d 11 (beta), Bos d 12 (kappa) | 19–25 each | Heat-stable — survives baking | Goat, sheep and buffalo milk (very high, over 90%); low with camel, donkey and mare milk |
Key Differences Between Casein and Whey Proteins
Casein (Bos d 8)
- Makes up 80% of the protein in milk.
- Heat-stable → Resistant to pasteurization and cooking.
- More allergenic and cross-reactive with other mammalian milks.
- Found in cheese, butter, yogurt, and processed foods.
Whey Proteins (Bos d 4, Bos d 5)
- Make up ~20% of milk proteins.
- More sensitive to heat and digestion.
- Found in protein supplements, infant formula, dairy drinks.
Cross-Reactivity with Other Foods
Milk Allergy Cross-Reactivity
- Goat, Sheep, Buffalo Milk: High cross-reactivity (casein proteins are highly conserved).
- Beef and Bovine Products: Possible reaction due to Bos d 6 (Serum Albumin).
- Soy Protein: Some people with milk allergy (especially infants with non-IgE allergy) also react to soy; this is usually a separate co-allergy rather than true cross-reactivity.
- Eggs, Peanuts, Wheat: Co-allergies are common but not due to direct cross-reactivity.
Clinical Implications
- Most people with milk allergy must avoid all dairy, including processed foods.
- Some can eat milk that is extensively heated (baked into foods), but this cannot be predicted by calling someone “casein-allergic” or “whey-allergic” and must be assessed by an allergist (see below).
- Severe Reactions → Strict avoidance of even trace amounts in foods.
Why Some People Tolerate Baked Milk
Caseins (Bos d 8) are heat-stable and survive baking, while whey proteins such as beta-lactoglobulin (Bos d 5) are partly changed by extensive heating. About 70–75% of children with milk allergy can eat milk baked into foods such as muffins. This cannot be predicted by calling someone "casein-allergic" or "whey-allergic": high casein IgE makes reactions to baked milk more likely, and tolerance must be confirmed by an allergist, often with a supervised challenge or a structured milk ladder.
Detailed epitope (amino-acid) maps previously shown here have been removed because they could not be matched to published studies. Exact epitope positions vary between studies and between patients.
Symptoms of Milk Allergy
Milk allergy can cause IgE-mediated reactions (immediate) or non-IgE reactions (delayed).
Immediate Reactions (IgE-Mediated)
- Hives, swelling, redness (urticaria, angioedema)
- Vomiting, diarrhea, abdominal pain
- Wheezing, coughing, difficulty breathing
- Anaphylaxis (severe cases, requires epinephrine)
Delayed Reactions (Non-IgE Mediated)
- Eosinophilic esophagitis (EoE)
- Food protein-induced enterocolitis syndrome (FPIES)
- Chronic diarrhea, reflux, colic in infants
Diagnosis of Milk Allergy
Skin Prick Test (SPT)
- Detects IgE sensitization to milk proteins. A positive result shows sensitization, not necessarily allergy; a larger result makes a true allergy more likely but does not predict how severe a reaction will be.
Serum IgE Testing
- Measures IgE antibodies to casein and whey proteins. Like skin testing, it shows sensitization rather than proving allergy; higher levels predict the likelihood of a true allergy, not its severity.
Oral Food Challenge (OFC)
- Gold standard for confirming milk allergy.
- Performed under medical supervision.
Atopy Patch Testing
Atopy patch testing has been tried for delayed (non-IgE) reactions, but it is not standardized and is not recommended for routine diagnosis. Delayed milk allergy is usually diagnosed by removing milk from the diet and then reintroducing it under medical guidance.
Anaphylaxis: act fast. After an exposure, any breathing symptom (wheeze, repeated cough, throat tightness, hoarse voice), fainting or dizziness, or symptoms in two or more body systems (for example hives plus vomiting) is anaphylaxis — even without hives.
- Give epinephrine right away (auto-injector or nasal spray). Do not wait to see if symptoms get better.
- Call 911, even if symptoms improve.
- Lie flat with legs raised (sit up if breathing is hard). Give a second dose after 5–15 minutes if there is no improvement.
Antihistamines and steroids do not treat anaphylaxis and must never delay epinephrine. Carry two doses and follow the written emergency action plan from your allergist.
Management and Treatment
Strict Milk Avoidance
- Avoid all dairy products, including hidden sources.
- Read labels carefully for casein, whey, lactose-containing ingredients.
Alternative Milk Options
| Milk Substitute | Suitable for Milk Allergy? |
|---|---|
| Soy Milk | Yes (unless allergic to soy) |
| Almond Milk | Yes (unless allergic to tree nuts) |
| Oat Milk | Yes (not a replacement for infant formula) |
| Rice Milk | Yes for older children and adults; not advised under age 5 (arsenic) and not a replacement for infant formula |
| Coconut Milk | Yes (unless allergic to coconut) |
| Goat/Sheep Milk | No (cross-reactivity) |
Emergency Treatment for Severe Reactions
- Epinephrine (auto-injector or nasal spray) for anaphylaxis.
- Antihistamines may ease mild skin symptoms such as itching or hives, but they do not treat anaphylaxis and must never delay epinephrine.
- Corticosteroids are sometimes given by doctors after a reaction, but they do not treat anaphylaxis and have not been shown to prevent a second-wave (biphasic) reaction.
Oral Immunotherapy (OIT) and Newer Treatments
- Milk oral immunotherapy is offered by some allergists to build up tolerance gradually. It is not FDA-approved for milk, carries a risk of reactions, and must be done under specialist supervision; a past anaphylactic reaction does not by itself rule it out.
- Omalizumab (Xolair), an injected medicine, was FDA-approved in February 2024 to reduce reactions after accidental exposure to milk and other foods. It is used alongside avoidance, not instead of it.
Comparison with Other Food Allergies
| Allergy | Key allergens | Clinically important cross-reactivity | Heat stability | Anaphylaxis possible |
|---|---|---|---|---|
| Milk | Caseins (Bos d 8), beta-lactoglobulin (Bos d 5) | Goat, sheep and buffalo milk | Caseins stable; whey proteins partly labile | Yes |
| Egg | Ovomucoid (Gal d 1), ovalbumin (Gal d 2) | Eggs of other birds | Ovomucoid stable; ovalbumin labile | Yes |
| Peanut | Ara h 2 and Ara h 6 (2S albumins), Ara h 1, Ara h 3 | Lupin; tree-nut co-allergy is common but mostly separate sensitization | High | Yes |
| Soy | Gly m 5, Gly m 6, Gly m 8; Gly m 4 (birch-related) | Clinical cross-reactivity with peanut is uncommon | Storage proteins stable; Gly m 4 labile | Yes (including Gly m 4 reactions to soy drinks) |
| Wheat | Tri a 19 (omega-5 gliadin), Tri a 14 (LTP) | Barley and rye (about 20% react clinically) | High | Yes (including exercise-induced) |
Summary of Milk Allergy
- Milk allergy is caused by casein and whey proteins.
- Casein is more heat-resistant and cross-reactive with goat/sheep milk.
- Symptoms range from mild to severe anaphylaxis.
- Strict dairy avoidance and alternative milk options are essential.
- Diagnosis involves skin tests, serum IgE, and food challenges.
Milk Protein Allergens: Common and Official Names
The FDA regulates milk as a whole food allergen; it does not list individual milk proteins. The official allergen names below come from the WHO/IUIS allergen database.
| Common name | Official allergen name |
|---|---|
| Caseins (total) | Bos d 8 |
| Alpha-s1-casein | Bos d 9 |
| Alpha-s2-casein | Bos d 10 |
| Beta-casein | Bos d 11 |
| Kappa-casein | Bos d 12 |
| Alpha-lactalbumin (whey) | Bos d 4 |
| Beta-lactoglobulin (whey) | Bos d 5 |
| Bovine serum albumin (BSA) | Bos d 6 |
| Immunoglobulins | Bos d 7 |
| Lactoferrin | No official allergen name (minor allergen) |
Key Insights
- Caseins (Bos d 8; individually Bos d 9–12) are the primary milk allergens, making up about 80% of milk proteins, and are highly heat-stable.
- Whey proteins (Bos d 4, alpha-lactalbumin; Bos d 5, beta-lactoglobulin) make up about 20% of milk proteins and are more heat-labile.
- Bovine serum albumin (Bos d 6) can cross-react with beef, so some people with milk allergy react to raw or undercooked beef. Immunoglobulins (Bos d 7) are minor allergens.
- Lactoferrin is a minor milk allergen, but it may contribute to reactions in sensitive individuals.