What kinds of allergy are curbed by what kinds of antihistamines?

It turns out that certain kinds of allergy aren’t helped by antihistamines at all! And it also turns out that every dog has a “favorite” antihistamine. Some dogs respond far better to Benadryl then they do to hydroxyzine and vice versa. Worse, there’s no way to tell. The full skinny on anti-histamine therapy for atopy and other allergies is in this recording.