This condition is caused by an allergic response to proteins found in a variety of fruits, vegetables, and nuts. It affects certain people who are allergic to pollen from the environment, particularly birch pollen in the case of fennel. As a result, some people who are sensitive to pollen may experience an immunological reaction if they eat raw or cooked fennel. Itching and burning sensations are localized to the mouth, lips, and neck in these persons. Symptoms might arise and then go quickly after eating or handling the problematic food. An anaphylactic response may develop in some more extreme situations. This response is not significant in the absence of additional symptoms, and fennel use should not be avoided on a regular basis. If you have comparable responses after eating fennel, you should see an allergist to figure out what's causing your allergies to plant foods. The latter will be able to determine whether further measures are required.