Dairy: This is the most common food allergy, with the most obvious symptoms. Doctors frequently misdiagnose milk protein allergies as lactose intolerance, which is a completely different problem entirely (and virtually nonexistant in children under age 4).
Symptoms of milk protein allergy include eczema, rough, patchy rashes, contact rash, mucus in stool, constant runny nose, wheezing, diaper rash with fine, red bumps, nausea, painful gas, and vomiting.
Soy: 40% of babies with a milk protein allergy also have a problem with soy, so it can be a good idea to cut both out when doing an elimination diet.
Symptomsinclude sudden, hives-like contact rash, gas, loose stool, and vomiting.
Egg: Egg was Jamie's worst allergy, and even a tiny bit of it in my diet was
enough to cause troubles for him. Egg is also contained within some vaccines.
Symptoms include rash, eczema, painful gas, loose stool, vomiting, and sudden, itchy contact rash.
Wheat: Signs of wheat allergy usually include eczema with a flat, red dotted rash and runny nose. If baby has a runny nose when dairy has been eliminated for >2 weeks, there's a good chance of wheat allergy.
Corn: Corn allergies can be extremely hard to diagnose (or eliminate once a diagnosis has been made) since it's in every-freaking-thing. Watch for corn syrup, HFCS, corn oil, etc. Symptoms include vomiting (either suddenly or delayed by up to an hour), contact rash, and eczema. Vomiting is the big one to look for.
Care needs to be taken when eliminating these foods from your diet, as they can be well hidden under other names (Whey and Casein also carry dairy) or could be well hidden in a food (i.e. something fried in butter, or brushed with egg)