How do I know if I have a food allergy?

What is a food allergy? 
An allergy is the immune systems response to a protein usually found in a food, such as peanuts, eggs, wheat or milk, that it perceives as a foreign body. These food allergies are usually acquired, not inherited. People may develop sensitivities to repeated exposure to certain foods in large quantities, or react to the chemicals in pesticides, herbicides, or otherwise genetically altered food.
How do I know if I have an allergy? 
The number of people in the US with food allergies varies. Some estimates claim as many as 1 in 4 people have some type of food allergy. Just because you don't exhibit obvious signs of a food allergy like hives or digestive problems doesn't mean you are allergy-free. The immune systems response to allergies can appear up to 72 hours after digestion, and in many different places throughout the body, so it is important to watch for these 13 warning signs