Following an allergic reaction some chemical mediators are released and among them histamine play a key role to develop various symptoms such as redness, itching, swelling, nasal secretion, breathing difficulty and  dizziness etc. In some cases it may cause serious systemic reaction (anaphylaxis) leading to collapse/death. For example, histamine gets attached to the receptors in blood antihistvessels, causing them to enlarge resulting in a sharp fall in blood pressure. 

Antihistamine works by inhibiting the action of histamine which help to control these symptoms. They are available in various forms such as tablets, liquid, injections, nasal spray, ointment, cream or eye drops. 
There are two types of antihistamines are currently in clinical practice. However, after the introduction of second generation H1 antihistamine in 1980, the first generation H1 antihistamine is being discouraged. First generation antihistamines are less effective and have an impact on the nervous system; which makes the patient drowsy, sedative in addition to some anticholinergic effects. Second and next  generations are more effective and have less adverse effects.
First generation H1 antihistamine such as diphenhydramine, chlorpheniramine, brompheniramine, and promethazine.
Second generation H1 antihistamines: are modification of first generation, relatively rapid onset and they are more effective and less or non-sedating and have no anticholinergic effects compare to first generation. Elimination half-life is as follows,  cetirizine (8 hours), loratadine (28 hours), terfenadine, mizolastine, astemizole
Next generation  antihistamine: Metabolite derivatives, safer, faster acting and more potent than second generation, fexofenadine (14 hours), levocetirizine, and desloratadine.