An allergy to lemons without a corresponding reaction to oranges is entirely possible. While lemons, oranges, limes, and grapefruits belong to the Citrus genus, their genetic differences mean they contain distinct combinations of proteins. An allergic reaction occurs when the immune system mistakenly targets a specific protein structure. The slight variations in protein composition between lemon and orange are significant enough to trigger a response to one fruit but not the other.
The Mechanism of Citrus Allergies
A true citrus allergy is an IgE-mediated reaction. The process begins when the body encounters a harmless protein, or allergen, within the citrus fruit and mistakenly identifies it as a threat. The immune system responds by producing a specialized antibody called Immunoglobulin E (IgE) specific to that protein.
These IgE antibodies attach to mast cells found throughout the body. Upon the next exposure, the protein binds to the IgE on the mast cells, triggering the release of inflammatory chemicals like histamine. Histamine causes the classic allergy symptoms, such as tissues swelling, itching, and constricting. Major allergens identified in citrus species include Cit s 1, Cit s 2, and Cit s 3.
Selective Allergy: Why Lemons and Oranges Differ
The possibility of reacting to lemon but not orange is rooted in the unique set of proteins each fruit contains. Citrus fruits possess several allergenic protein families, including profilins (Cit s 2) and Lipid Transfer Proteins (LTPs). While profilins are shared across many plant foods, LTPs often cause more severe, localized allergies.
The key distinction lies in the specific structure of these proteins, which varies between species. For example, the LTP found in lemon is Cit l 3, while the analogous protein in orange is Cit s 3. Subtle differences in their amino acid sequences mean that an individual’s IgE antibodies may only recognize the unique shape of Cit l 3 in lemon. This selective recognition allows a person to consume an orange without issue, while a lemon triggers an allergic response.
This high specificity differentiates a selective allergy from general cross-reactivity, where the body reacts to multiple related fruits due to shared protein structures. The concentration of these proteins also varies between the pulp, peel, and seeds. A person might only be sensitive to the lemon peel, for instance, which contains a higher concentration of certain allergens.
Identifying Symptoms and Getting a Diagnosis
Symptoms of a citrus allergy can range from mild local reactions to severe systemic responses. A common presentation is Oral Allergy Syndrome (OAS), where proteins in the raw fruit cause immediate itching, tingling, or swelling of the lips, mouth, tongue, and throat. Contact with the fruit’s peel, which contains allergenic oils, can also lead to contact dermatitis, characterized by skin redness, burning, and itching.
More serious reactions may involve gastrointestinal symptoms like nausea, vomiting, or abdominal pain. In rare cases, a person may experience anaphylaxis, a life-threatening systemic reaction that can include widespread hives, difficulty breathing, or a drop in blood pressure. If an allergy is suspected, an allergist will begin the diagnostic process with a thorough medical history and physical exam.
Diagnosis is often confirmed using objective tests. A skin prick test applies a small amount of citrus extract to the skin to check for a localized hive. Blood tests can also measure the level of specific IgE antibodies reacting to citrus proteins. In some cases, an oral food challenge, performed under medical supervision, is used to confirm the allergy diagnosis.
Managing a Citrus Allergy
Managing a confirmed citrus allergy involves strict avoidance of the specific fruit and any derived products. This requires careful reading of ingredient labels, as citrus components are often used as flavorings or preservatives in processed foods and beverages. Individuals must also be mindful of non-food sources, as citrus extracts and essential oils are commonly found in cosmetics, lotions, and cleaning products.
For accidental or mild exposures, over-the-counter antihistamines can help alleviate symptoms like itching and hives. Individuals diagnosed with a risk of severe reactions must carry an epinephrine auto-injector and have an emergency action plan developed with their allergist. Knowing how to properly use this device is essential for emergency preparedness.