As we know, allergic reactions involve an interaction amongst IgE, allergens, mast cells or basophils, which release chemical mediators such as histamine, leukotrenes, and  prostaglandins to name a few.  After being released from mast cells, histamines seek out  "receptor" sites located in the nasal lining tissue.  Like a key inserted into a lock, the histamine (key) links up with a receptor site (lock) and triggers allergic symptoms such as sneezing and itching.

First generation antihistamines (over-the-counter) have been used to treat allergic symptoms since 1942.  Antihistamines counter the effects of histamine because they attach themselves to the receptors before histamine gets to them.  Because receptors accept only one chemical at a time, if antihistamines block histamine, allergic symptoms won't be triggered.

The most common antihistamine medications are first-generation or nonprescription products available over-the-counter.  Most of these products, while bearing different brand names, contain similar active ingredients such as Brompheniramine maleate (Dimetapp), Chlorpheniramine maleate (Chlor-Trimeton), Clemastine fumrate (Tavist-1), and Diphenhydramine hydrochloride (Benadryl).

Although first-generation OTC antihistamines can relieve symptoms of allergic rhinitis, they also can make you drowsy by crossing the blood-brain-barrier and affecting histamine receptors in the central nervous system, thus resulting in drowsiness.  Other side effects of first generation OTC antihistamines can include dry mouth, gastrointestinal distress, and urinary retention.

Due to noteworthy advances in research, several newer, second-generation anti-histamines have been developed such as Zyrtec, Allegra, Claritin, Clarinex, and Alavert that do not cross the blood-brain-barrier, therefore are less sedating, if sedating at all.  Also, the occurrence and severity of other side effects are much less than first generation antihistamines.  The only downfall of second generation antihistamines is that they are normally more expensive.

Typically, antihistamines work best when taken on a regular basis before exposure to the offending allergen.  So, if you know that animal dander triggers your allergic rhinitis and you plan to visit someone who has pets, take your anti-histamine two to five hours beforehand.

Because antihistamines normally do not decrease nasal congestion, they are frequently combined with a decongestant (D) such as Allegra-D, Zyrtec-D, and Claritin-D.  The most commonly used decongestants are pseudoephedrine and phenylephrine.  Common side effects of oral decongestants are sleeplessness, nervousness, agitation, dryness of the mouth, difficulty urinating, high blood pressure and heart palpitations.

Even though both first and second-generation antihistamines are quite effective in controlling or alleviating symptoms of sneezing, runny and itchy nose, and throat itching, other chemical mediators such as leukotrene and prostaglandins play a big role in allergic rhinitis, especially nasal stuffiness.