Oral allergy syndrom, continuation of continuous consumption of allergen
When you ingest an allergen for the first time, your body produces many of an antibody called IgE with specificity toward that allergen. These IgE antibodies bind to mast cells, and basophils. When you encounter more allergen, they bind to these IgE on the surface of the mast cells and basophils, causing them to release a host of chemicals like histamine. Massive amount of histamine are the cause of anaphylaxis, so when more allergen interacts with more and more IgE, the situation worsens. The following figure is a detailed mechanism,



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We can also see, though, that histamine released is a function of total allergen, and eventually reaches a maximum regardless of allergen present,



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Here they're showing histamine release vs allergen concentration while investigating allergic rhinitis.