People who are allergic to avocadoes are prone to the same types of allergic symptoms as people who are allergic to other foods. Likely reactions include hives, itchy eyes or nose, runny nose, congestion, asthma, stomach cramps, difficulty breathing, swelling, vomiting and occasionally life-threatening anaphylaxis. These result from an immune system response to certain proteins in avocadoes; your body thinks they're harmful, even though they're not. In response, your immune system releases an antibody called immunoglobulin E. It triggers chemicals that include histamine, which is what causes your allergic symptoms.
Other foods likely to cause reactions in people with avocado allergies are apples and kiwi fruits. In addition, people with latex allergies might react to avocadoes, too, since some of the same allergens exist in both. Symptoms can often be treated with over-the-counter medications, but the best way to prevent an allergic reaction to avocadoes is to avoid them altogether.