Short answer: histamines cause allergies' reactions :-)
Long answer? Allergies are responses to things in our environment; whether they're ingested, exposed on contact, or breathed in, they all indicate roughly the same thing -- our immune system isn't happy with whatever it's been exposed to.
General thought is that we all have items we're predisposed to be sensitive to; these can be "turned on" with the first exposure, such as children with many food allergies, or they can be allergies that we "grow in to," via repeated exposure (such as with many latex allergies).
An allergic response (the body's attempt to get rid of the offending material) is generally considered to be local (for instance, a skin-rash at the site of exposure to an allergen such as latex) or systemic (an all-over body response that can included obstructed breathing as part of its repertoire). Local allergies can graduate to systemic allergies, as in the case of my best friend, an RN who's been in the military and deployed many times. Her contact allergy to many of the treated tools she encounters daily developed into a systemic response to any and all formaldehydes -- whether she's exposed by touch, or by breathing or ingesting them.
Was there a particular response/reaction or allergen you were inquiring about?