Allergic reactions from eating bananas is thought to be caused by chitinase, which is a plant derived enzyme made by the plant naturally to act as a defence against fungal attacks.
Banana allergy is most often seen in conjunction with allergies to other foods, either presenting as Pollen-Food Allergy Syndrome or Latex Food Syndrome.
Some of the linked allergies are in foods which contain high levels of chitinase, like chestnuts, kiwi, avocados, papaya, and tomatoes.
If sensitised to Ragweed pollen you may also have problems with Melon, Courgette, Cucumber, and Squash
45% of latex allergy sufferers are also allergic to bananas.