Milk allergy is an adverse immune reaction to one or more proteins in cow's milk. Symptoms may take hours to days to manifest, with symptoms including atopic dermatitis, inflammation of the esophagus, enteropathy involving the small intestine and proctocolitis involving the rectum and colon. However, rapid anaphylaxis is possible, a potentially life-threatening condition that requires treatment with epinephrine, among other measures.

In the United States, 90% of allergic responses to foods are caused by eight foods, including cow's milk. Recognition that a small number of foods are responsible for the majority of food allergies has led to requirements to prominently list these common allergens, including dairy, on food labels. One function of the immune system is to defend against infections by recognizing foreign proteins, but it should not overreact to food proteins. Heating milk proteins can cause them to become denatured, losing their three-dimensional configuration and allergenicity, so baked goods containing dairy products may be tolerated while fresh milk triggers an allergic reaction.

The condition may be managed by avoiding consumption of any dairy products or foods that contain dairy ingredients. For people subject to rapid reactions (IgE-mediated milk allergy), the dose capable of provoking an allergic response can be as low as a few milligrams, so such people must strictly avoid dairy. The declaration of the presence of trace amounts of milk or dairy in foods is not mandatory in any country, with the exception of Brazil.

Milk allergy affects between 2% and 3% of babies and young children. To reduce risk, recommendations are that babies should be exclusively breastfed for at least four, preferably six months, before introducing cow's milk formula. If there is a family history of dairy allergy, substitutes like extensively hydrolysed, non-dairy or elemental formula may be discussed. Soy infant formula is common, but about 10 to 15% of babies allergic to cow's milk will also react to soy. The majority of children outgrow milk allergy, but for about 0.4% the condition persists into adulthood. Oral immunotherapy is being researched, but it is of unclear benefit.

Food allergies can be classified as rapid-onset (with symptoms manifesting within minutes to an hour or two), delayed-onset (up to 48 hours) or combinations of both, depending on the mechanisms involved. The difference depends on the types of white blood cells involved. B cells, a subset of white blood cells, rapidly synthesize and secrete immunoglobulin E (IgE), a class of antibody that binds to antigens, the foreign proteins. Thus, immediate reactions are described as IgE-mediated. The delayed reactions involve non-IgE-mediated immune mechanisms initiated by B cells, T cells and other white blood cells. Unlike with IgE reactions, there are no specific biomarker molecules circulating in the blood, and confirmation of the allergy is achieved by removing the suspect food from the diet and determining if symptoms dissipate as a result.

IgE-mediated symptoms include: rash, hives, itching of the mouth, lips, tongue, throat, eyes, skin, or other areas, swelling of the 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. Symptoms of allergies vary from person to person and may also vary from incident to incident. Serious allergic danger can begin when the respiratory tract or blood circulation is affected. The former can be indicated by wheezing, a blocked airway and cyanosis, the latter by weak pulse, pale skin and fainting. When these symptoms occur, the allergic reaction is called anaphylaxis, which occurs when IgE antibodies are involved and areas of the body not in direct contact with food become affected and show severe symptoms. Untreated, this can proceed to vasodilation, a low-blood-pressure situation called anaphylactic shock and very rarely, death.

For milk allergy, non-IgE-mediated responses are more common than are those that are IgE-mediated. The presence of certain symptoms, such as angioedema or atopic eczema, is more likely related to IgE-mediated allergies, whereas non-IgE-mediated reactions manifest as gastrointestinal symptoms, without skin or respiratory symptoms. Within non-IgE cow's milk allergy, clinicians distinguish among food protein-induced enterocolitis syndrome (FPIES), food protein-induced allergic proctocolitis (FPIAP) and food protein-induced enteropathy (FPE). Common trigger foods for all are cow's milk and soy foods (including soy infant formula). FPIAP is considered to be at the milder end of the spectrum and is characterized by intermittent bloody stools. FPE is identified by chronic diarrhea that dissipates when the offending food is removed from the diet. FPIES can be severe, characterized by persistent vomiting one to four hours after an allergen-containing food is ingested, to the point of lethargy. Watery and sometimes bloody diarrhea can develop five to ten hours after the triggering meal, to the point of dehydration and low blood pressure. Infants reacting to cow's milk may also react to soy formula, and those reacting to soy formula may react to cow's milk. International consensus guidelines have been established for the diagnosis and treatment of FPIES.

Conditions caused by food allergies are classified into three groups according to the mechanism of the allergic response: