So, we first figured out that my first child had a dairy allergy when we tried to give him formula when he was about 5 months old. Previously, he had only had mommy juice (aka nursing). The first time we gave him formula was when I mixed it with rice cereal. Within 10-15 minutes he started getting blotchy. Red welts (which I later learned were hives) on his face and hands ... everywhere the rice cereal touched. Thankfully he didn't have breathing problems or swell up to twice his size, but it was definitely scary. At that time, I was a fairly new mother, so anything unusual puts you into hyperdrive.
Let me digress a minute and just say that as time went on and I got more used to dealing with effects of food allergies, my reaction was very different when those red welts would pop up. As long as there are only a couple hives, now I often just sort of shrug it off. If my son is not bothered it by, then I'm not. I think about what could have possibly been in the food he just ate, but many times I can't figure out what could have caused it. Either way, I might give him a dose of Benadryl, but often I just wait it out.
So, anyway ... we called the pediatrician and he suggested we try the other brand of formula -- same thing. So then we tried soy formula and ta-da, he was okay. We stuck with soy formula and assumed he had a milk allergy, but weren't totally sure.
Anyone out there who wants to share their "first time" they saw an allergic reaction??