I'm not sure whether there actually is an allergy against alcohol. Generally, an allergy is an exaggerated response by your immune system to a substance. Basically, your body thinks, some substance (mostly proteins) is part of a pathogen (like a hull protein of a bacterium) and responds accordingly. There are different types of allergies depending on what part of your immune system gets activated.

The adverse effects of alcohol on the other hand, are mostly effects of the poisonous byproducts of intermediate products of alcohol metabolism (the process in which your body gets rid of the alcohol).

Of course, you can be allergic to another compound of an alcoholic beverage, like some proteins from the malt used to make beer or have another kind of reaction, which isn't an allergy to other compounds.