The symptoms of an allergic reaction to food can range from a slight discomfort to a fatal reaction. These include:
Oral Allergy Syndrome: This is a discomfort in the mouth akin to an itchy or scratchy feeling. It is easily treated in a short while after food is either swallowed or removed from the mouth.
Food protein-induced enterocolitis syndrome: This results in vomiting and diarrhea.
Eosinophilic esophagitis: The symptoms here are a difficulty in feeding and loss of weight in babies; and a reduced appetite, vomiting, heart burn, and difficulty in swallowing food in children and adults.
Anaphylaxis: This results in a rapid drop in blood pressure, inability to breathe, swelling in different areas all over the body, and throwing up food. It can turn life threatening and requires immediate self medication by injecting epinephrine and calling for professional medical aid.
The information here has been referenced from the link below, which should be referred for complete details.