Food allergies range from very mild to life-threatening. The mildest symptoms are vague itchiness in the mouth and throat. Other mild to moderate symptoms:

• general itchiness
• hives or rash, sometimes all over body
• runny/itchy nose and eyes
• recurrent earaches
• nausea and vomiting
• diarrhea
  

Some foods can cause a life-threatening anaphylactic reaction. The mouth, throat, and bronchial tubes swell enough to impede breathing. The person may wheeze or faint. Often there are generalized hives and/or swollen face. This is an emergency!! As anyone would, call your doctor or 911! For breathing trouble or loss of consciousness, call 911 immediately. See also the sections on insect stings and anaphylatic reactions.

One severe allergic reaction to food puts you at risk for more. Discuss with your doctor what to do for repeat reactions.
Common food allergens:
peanuts: This is often life-threatening. Call a doctor for ANY reactions to peanut products! (Peanuts can be a hidden ingredient in a number of foods.)
soy: again can be a hidden ingredient in a number of foods.
fish and/or shellfish: in some people, these reactions can be severe, ranging from extreme nausea to breathing difficulties. Watch carefully and call 911 for any breathing problems.
berries
peppers
milk proteins: (less common than you'd think - most people are intolerant not allergic).
wheat (and gluten), as well as some other grains (corn, rye)
eggs
many reactions have unknown cause!
Interestingly, some common food proteins are similar enough to ragweed to cause reactions in sensitive people. These include bananas and melons.
Allergic reactions may progress from mild to severe, so keep track of any reactions.
Food allergies may be amount-sensitive. That is, you don't feel the reaction until you've ingested a certain amount; however, severe reactions may occur with ANY tiny amount of allergen. This is especially true of peanut allergies.