eli5. Why is it that you don’t have a reaction to something for the first time if you are allergic to it?

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For example if you eat something that you are allergic to for the first time, nothing will happen. But the second time you’ll have a reaction.

In: Biology
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An allergic reaction is an inflammatory response as part of your immune system. While your immune system is good at fighting off new antigens, where it excels is with secondary exposures. Memory cells and antibodies are ready and will initiate that cytokine cascade and the inflammatory response.

Edit: wording is hard

There’s two phases of an allergic reaction. The sensitisation phase and the reaction phase

1. Sensitisation: this occurs upon first exposure to the allergen. For very complex reasons your body decides that the allergen is dangerous and tells your B-cells (a type of cell involved in your immune response) to prime your mast cells (another type of cell involved in immunity). This is pretty much like setting up a trip wire for a mine that’ll only go off when you encounter the allergen for a second time. That brings us to…

2) reaction: this happens bc the allergen sets off that trip wire causing your mast cells to release a ton of pro-inflammatory and other molecules resulting in the allergic response.