How do hives appear so quickly after you eat something you’re allergic to?

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Today I accidentally ingested something that I’m mildly allergic to, and within minutes I had hives on my shoulders and scalp (which is my usual reaction).

I understand that hives are caused by your immune system being activated. What I don’t understand is: if you’ve just ingested the food you’re allergic to, how is it that your immune system gets activated before you’ve had any time to digest it? And why do hives appear on the scalp, which is far away from where the food would be absorbed?

In: Biology
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Digestion starts in your mouth! As you chew, saliva gets mixed in, which has white blood cells in it among other things. Those are able to transmit an immune response very quickly

People with allergies have heightened immune response to these allergens that can be activated with the smallest of triggers… be it smell, taste or touch… That is why asthmatics have instantaneous reactions to dust or wierd smells… Same happens to allergic food that is ingested… Also this immune reaction occurs in the blood, and so all the effects pertaining to this reaction can occur wherever the blood flows! Be it scalp, lungs, hands, legs.

Hives are caused by whatever you ate binding to an antibody that hangs out in your blood that activates a special kind of immune cell, called a “mast cell.” When the mast cell is activated, it triggers a bunch of other immune cells to activate and releases chemicals like histamine that make your blood vessels leaky, allowing immune cells and fluid to get from vessels into the peripheral tissues, like the skin of your scalp, causing hives and swelling. This all happens super fast.