so is histamine in allergies, does that mean that allergies or exposing oneself to allergens automatically drop blood pressure and what if it is only a slight allergen, does blood pressure still go down? i know histamine rebalances fluids and electrolytes but to what extent and how much of an allergic reaction is needed before blood pressure falls significantly?
Excellent question, but you kinda answered it yourself already.
If it is a mild allergic reaction, then it would depend on what type* of allergic reaction, and specifically if you are talking pets or seasonal or food. The internal response to a food* you are allergic to will usually yield more internal effects on a set time range, and can be very severe, depending on how allergic you are...Seasonal and pet allergies may evoke a skin mediated reaction quicker, but both are about equivalent in terms of effects on hydration status - but again, you are talking apples to oranges here, there will be variation based on YOU and HOW allergic you are, and your susceptibility to allergic responses is also determined by other factors such as stress (which releases histamine to trigger cortisol response), diet and exercise frequency.
Taking a walk even, will raise histamine decarboxylase - and thus more histamine production...and most cardio/calisthenic style workouts trigger histamine response faster than slow movement workouts...unless you are using a lot of weight and it becomes a core body or resistance style workout. These are healthier variants of histamine response however.
Brain and neuronal histamine do not necessarily follow the same rhythm as peripheral histamine release; meaning - you could have high histamine in the brain and nerves but not in the skin or elsewhere...this is entirely possible. You can also have low histamine in the brain and tons in the periphery, although this one is rare and more obscure by contrast.
Keep in mind that histamine is also not the only vasodilator, it simply acts as one (a major one though) out of many nitric oxide promoting chemicals, and also plays a role in oxytocin release and vasopressin release. Generally speaking, the more histamine that builds up, the less vasopressin but more oxytocin, which explains why many high histamine people "wear their heart on their sleeve" and can be over emotional...
High histamine people, partially due to the low vasopressin levels , also tend to be very lean, defined, cut up etc and veins are prominent. So they literally wear their heart on their sleeve, both in the physical sense and psychological one.