That's a nice explanation, ksfrog! But you made histamine sound negative - one thing I don't like about anti-histamine meds is that it's hindering a natural process - thus, when taking them, you might also impede immune reactions against natural pathogens/infections. Histamine helps dilate and permeablize blood vessels so that immune defense cells can mobilize and do their work.
I'd like to add that most allergies are the eventual results of acquiring foreign proteins (pollen, pet dander, dead foreign skin, etc.) directly into your bloodstream in a concentration high enough to provoke an immune reaction. An ensuing arsenal of memory immune cells later roam the body, armed with antibodies to attack the foreign proteins, giving you a permanent, acquired allergy.
That's why you don't feed newborn infants dairy milk - because the pH (acidity) of their stomach is too bassic, allowing the dairy milk to pass straight through the stomach undigested into the intestines and then into the bloodstream. The undigested milk proteins trigger an immune reaction and as the baby grows up, he/she will become severely allergic to all dairy products the rest of his/her life. As babies age, their stomach pH levels change to a point where they can digest dairy products and it'd be ok to feed it dairty products then. Same goes with skin creams containing peanut oil - careless mothers who use too much of this can actually cause some of the peanut oil/extracts to leak into the bloodstream of the infant through the skin (or perhaps the babies lick it off their skin/fingers?) and the child may acquire a lifelong peanut allergy.
Interesting, I'd say!
EDIT: On the technical side, ksfrogman - if I was allergic to my girlfriend, it wouldn't be the work of a typical antibody-dependent immune reaction since we're both the same species and have the same proteins...reaction against her dead skin cells would be based off of MHC receptors, leading T cells to do much of the dirty/killing work...analogous in transplant rejections.
