– Why do you have headaches after or days after an allergic reaction? Again, it’s the bodies
immune system responding to what it believes is a foreign threat. It gets out of balance, out of control, and you get vasodilatation. One of the main moderators
of allergic reactions, especially the immediate
phase, and can lasting after, is histamine, and histamine
and vasodilatation is related to promoting or triggering headaches, especially what we call
quote-unquote migraine headaches. And so it’s sort of a delayed response to the body getting out of balance. Histamine and vasodilatation
and then blood flow being shunted from one part
to another part of the brain, which causes a headache, the pain.