Allergic reactions to plant foodstuffs comprise a whole spectrum of clinical manifestations, from very mild ones, localized to oral mucosa, to the most severe including life-threatening anaphylactic shock. The reason is the presence of a few structurally and functionally different allergens in each foodstuff. On the other hand, the similarity between particular allergens in different foodstuffs and the presence of their homologues in other sources (pollens, insect venoms) give a lot of possibilities for cross-reactions during allergic sensitization. Apple has a significant place in human nutrition. However, it is also a fruit that most frequently causes allergic reactions. Apple allergens are well known and appropriately characterized, and their homologues are present in other vegetable foodstuffs. Thus, apple represents a good model of plant food hypersensitivity. The aim of this paper is to review the mechanisms of allergic sensitization, the types of clinical manifestations, the influence of allergen content variability, and the diagnostic and therapeutic issues connected with plant food allergy, using apple as a model.