There are several types of food antigen reactions. 

Immunoglobulin E (IgE) reactions are commonly referred to as “true allergies”. They produce symptoms like wheezing, runny nose, hives, itchiness and even anaphylactic shock. IgE symptoms come on quickly, within minutes to hours, making it easy to identify what caused the reaction. About 5-10% of the population experience IgE reactions to foods.
 
Immunoglobulin G (IgG) reactions to food, on the other hand occur in an estimated 80% or more of the population.  Symptoms for IgG reactions include muscle aches, joint pain, headaches, insomnia, cravings and fatigue making it often difficult to identify the root cause.  The symptoms are both long-lasting and delayed, occurring as a result of immune complexes depositing and triggering inflammation responses. Taking days to develop and having a 23 day half-life, it is quite difficult to ascertain which foods are causing IgG symptoms.