Histamine is a chemical (neuro-transmitter) responsible for numerous symptoms of allergic reaction. The amount present in the blood or urine can help confirm or rule out the presence of an allergy. Although there is always some histamine in your body, a mosquito bite (for example) causes your body to release more histamine in the area of the bite, making your skin red and itchy. In extreme cases, histamine levels in someone who is allergic to a bee sting or a particular food like peanuts can be elevated so high that it causes anaphylactic shock and possibly death.
Adrenaline (Epinephrine) is the only chemical that can quickly eliminate histamine in a person. So called "antihistamines" like Benadryl only work to block some of your body's histamine receptors (relieving some histamine related symptoms), they do not remove histamine.
Depending on the results, further testing may be recommended. Possibilities are: skin tests, RAST, or challenge tests; biopsy and/or CT scan.