An egg allergy is an adverse immune system response that occurs when the body mistakenly identifies egg proteins as harmful. While most allergic reactions are triggered by proteins in the egg white, it is possible to have a specific allergy to just the egg yolk. This less common allergy involves a reaction to the unique protein components within the yolk, distinguishing it from typical egg white sensitivities.
Distinct Proteins in the Egg
The egg’s division into two distinct parts provides the biological basis for why an allergy can be specific to either the white or the yolk. Egg white contains the highest concentration of allergens, including four major proteins: ovalbumin, ovomucoid, ovotransferrin, and lysozyme. Ovomucoid is considered the dominant allergen due to its resistance to heat and digestive enzymes.
The yolk is generally considered less allergenic but contains its own unique proteins that can trigger an immune response. The major allergen identified in the yolk is alpha-livetin (Gal d 5), a form of chicken serum albumin. Other allergenic proteins in the yolk include apovitellenins and phosvitins.
An allergy solely to the egg yolk is often associated with Bird-Egg Syndrome, typically seen in adults but also occurring in children. In this condition, the individual is first sensitized to airborne bird proteins, such as those in feathers or droppings, which contain serum albumin. The immune system then recognizes the closely related alpha-livetin in the egg yolk, triggering an allergic reaction upon ingestion.
The heat stability of the proteins involved determines whether a person can tolerate cooked egg yolk. Alpha-livetin is a heat-labile protein, meaning cooking easily breaks down its structure. Therefore, individuals with a yolk-only allergy focused on alpha-livetin may be able to safely consume thoroughly cooked egg yolk, such as in baked goods.
Recognizing Allergic Reactions
An allergic reaction to egg yolk is a rapid response mediated by the immune system’s production of Immunoglobulin E (IgE) antibodies. Symptoms typically start within minutes to an hour after consuming the allergenic protein. Immediate IgE-mediated reactions can manifest across multiple body systems.
Dermatological symptoms are common and may include hives, generalized itching, or swelling of the face, lips, tongue, or throat. Gastrointestinal issues frequently occur, presenting as vomiting, abdominal pain, or diarrhea. Reactions specific to egg yolk may be more likely to involve gastrointestinal symptoms and less likely to cause respiratory issues compared to egg white reactions.
Respiratory distress can range from a runny nose and sneezing to wheezing and difficulty breathing. Anaphylaxis, a severe and potentially life-threatening systemic reaction, is a risk with any food allergy, including a yolk-only allergy, and requires immediate treatment with epinephrine.
Egg proteins can also cause non-IgE-mediated or mixed allergic reactions, which have a delayed onset, sometimes hours or days after ingestion. These reactions often present as a flare-up of atopic dermatitis or involve chronic gastrointestinal symptoms. Observing the pattern and timing of symptoms after consuming egg is an important first step in understanding the sensitivity.
Confirming the Allergy and Dietary Steps
A suspected egg yolk allergy requires consultation with an allergist, who uses medical history and diagnostic tests to confirm the condition. The initial evaluation often involves skin prick tests (SPT) and specific IgE blood tests (sIgE). These tests can sometimes differentiate sensitivity to the overall egg from sensitivity to specific protein components, such as alpha-livetin (Gal d 5) found in the yolk.
While skin and blood tests indicate sensitization, a positive result does not always equate to a clinical allergy. The gold standard for confirming a food allergy is the oral food challenge (OFC), conducted in a medical setting under direct supervision. During this procedure, the patient consumes gradually increasing amounts of the suspected allergen to confirm a reaction or tolerance.
Once a yolk allergy is confirmed, primary management involves strict avoidance of the allergenic part of the egg. Separating the yolk from the white without cross-contamination is nearly impossible, making complete egg avoidance the safest choice for many individuals. It is necessary to carefully read food labels, looking for terms like “egg yolk,” “livetin,” and “albumin,” as complete separation is difficult during food processing.
If the allergy is confirmed to be solely to a heat-labile protein like alpha-livetin, the allergist may recommend a trial of extensively cooked egg yolk. For individuals who tolerate cooked egg, this differential avoidance can significantly improve dietary options and quality of life. Regular follow-up with the allergist is recommended, as the severity of the allergy and the degree of tolerance can change over time.