More info - here's our experiences that lead to a diagnosis. Pls forgive me, for I may need to edit and add things in later. I posted this on another thread and thought it would be good here.
When we introduce foods, we do it 2x a day for 3 days. DD's reactions lead us to testing.
I knew something was wrong from early days and it wasn't until she was 10 months that it was confirmed. Mysterious dot rashes from 5 months old plus below info. Bottom line: trust your instincts as when to see an allergist and if you need to change allergists (we did)
DAIRY
At a few day's old dd projectile vomited for like 30 min every time she had formula (milk based like enfamil). We were supplementing my breastmilk. We switched to soy (isomil) and had no problems.
At 10 months we did a tablespoon or 2 of cottage cheese. She started vomiting 2 hrs later. She vomited non stop - and i mean literally non-stop, for 4 hours. I couldn't even leave the bathroom sink i was holding her over. Finally it was some blood too. She was so lethargic, she couldn't even hold her head up. Ped thought it could have been the consistency so they wanted us to try again 1 month later w/ yogurt. We tried again 1 month later w/ 1 TBS of Yo Baby drink. Well, 2 hrs later she vomited the same way for 3 hrs w/ same symptoms. ETA: In retrospect, had I known more even despite dr's orders, with the 1st major reaction, I shouldn't have tested it at home the 2nd time. We're lucky she's ok.
She was diagnosed by the allergist w/ dairy allergy and milk protein induced enterocolitis (MPIES) mainly by reactions. She tested positive for dairy on the blood test w/ low numbers. I think her skin test was negative. If it wasn't for the vomiting, she'd be not "allergic". They think MPIES is what caused the vomiting - the gastro system shuts down and all bodily fluids rush to the stomach (and colon i believe) and out. Other foods can cause it too in people (FPIES). There's no tests to confirm MPIES/FPIES, only reactions. Usually the reaction is 2 hrs or so after eating the food.
ETA: Skin test currently negative, blood test number still low, but up a little bit. Could challenge now if it wasn't for the MPIES.
When she's age 3+ , if her skin test and blood numbers are ok, we can food challenge dairy. However, it needs to be in the hospital w/ an IV already in place. She needs to be over 3 to even test it to be able to handle the potential vomiting physically, mentally, & emotionally. That's how serious and dangerous the vomiting is. It literally could have killed her b/c of quick dehydration. They veins collapse too, so an IV is very difficult.. An epi pen wouldn't have stopped it.
EGG
Vomited on day 1 of egg yolks/second exposure right after having it, but it only was a few seconds. Day 3 we also got the flu shot. 45 min later, full body hives. She tested positive on skin and blood. Diagnosed egg allergy due to tests and reactions. Outgrew egg at age 2. Skin and blood tests were negative so we did an oral food challenge in the dr's office, which she passed.
FISH/SHELLFISH
Skin tested positive with 1 allergiest (we see different one now), but she never ate it so it can't be confirmed. Didn't do blood at age 1. Recent tests (b/c of a food allergy study) were negative for blood/skin. Not introducing anyway until age 5 and numbers can change. Protectively avoiding. ETA: You're not supposed to blood or skin test something you haven't eaten - partly b/c you can't confirm issues w/o personal history with it.
PEANUTS/TREENUTS
Had mysterious reaction several times (severe eczema, diaper rash and screaming etc) to 365 Rice Milk. Didn't test back then b/c it wouldn't matter. Not introducing until 5. Plus pn/tn weren't in the product, so it could only be possible cross contamination of the oils. Also had a hive, red swollen eyes & rubbing eyes when we were in the bulk peanut etc aisle at whole foods (ETA: peanut/tree nut dust). Stopped screaming when we left the area. ETA: Went in aisle again and she was very nervous etc, no one in the aisle. Left area and issues stopped.) Recent tests (food study) were negative to skin, tiny numbers to blood. Numbers change w/ age though. Highly suspect, avoiding as an allergy.
DRUGS: Sulfas, penicillins, erythromycins - completely miserably w/in a day or so, full body rash by day 7. No tests available. Many drugs have dairy or other top food allergies in them.
We also avoid seeds because sesame is the top 9th allergen. Dairy, soy, wheat, peanut, tree nut, fish, shellfish, egg being the top 8, in no particular order. Sulphites is 10th. With seeds though, we don't worry about shared lines or anything, we just don't give her concentrated seeds or seed oils (flax for example). She can have mustard if she likes it, but she doesn't.
Possibly more than you wanted to know.