I'm not sure specifically as there could be many novels with that name. It might be about a unique love story involving an allergen in some way, perhaps a character has an allergic reaction that sets off a series of events in the plot.
It's possible that there are multiple characters, each with their own relationship to the concept of the 'allergen'. Maybe a family where one member has an allergy and the others have to adjust, and the story is about their journey together.
In a lovely love story, a plot could be that a person moves to a new city and feels lonely. One day, they join a local community event. There, they meet someone who makes them feel welcome and at home. They start to hang out more often, sharing their past experiences and future hopes. Their love story is about finding love and a sense of belonging in a new place.