Food allergies are what happen when your child's immune system makes a mistake. It thinks the proteins in certain foods are dangerous even though they're not. So when your kid eats something he's allergic to, his body wages war. Antibodies are sent out, and they alert the mast cells to take action. The mast cells trigger a team of chemicals, which set out to fix the problem. One of these chemicals in particular - histamine -- is what causes the allergic symptoms that made you suspicious that your child had an allergy in the first place.
A doctor can test a child for food allergies in a number of ways; but before he does any testing, he'll take a medical history. The doctor will want to know how long it took for your child to react after he ate a suspected food and what type of reaction he had. He'll also want to know if any other family members have food allergies. If the allergist decides an allergy might be at fault for your child's reaction, he'll probably try a skin test, blood test or elimination diet.
In a skin test, the doctor applies small extracts of possible allergens to your child's arm or back; then he scratches or pricks the skin lightly so that the extract can seep in. If a small bump develops on the site of a scratch, it's likely that your child is allergic to that allergen. For a blood test, the doctor draws blood and sends it off to a lab where it's mixed with various allergens and then tested for a certain antibody. If the antibody is present, an allergy is probable. With an elimination diet, the doctor will tell you which foods to avoid giving your kid over the course of the next few weeks. One by one, foods are reintroduced. As each food is reintroduced, you have to keep an eye on your child to see whether he has any allergic reactions.