There is an antibody, Immunoglobulin G (IgG), that can also cause allergic responses but the reactions are delayed.   This delay may make it difficult to identify the specific allergen that is causing the reaction and the discomfort may not be recognized as an allergy.  An IgG response may not occur until hours or even days after exposure to the allergen.  Symptoms may include headache, nausea, irritable bowel syndrome or other digestive issues, seizures or hyperactivity, making diagnosis difficult.

Not so long ago people, including many children, were subjected to hundreds of needle pricks containing different possible allergens to determine which ones caused the strongest reactions but this method is ineffective for IgG allergies due to the delayed reaction.  A simple blood draw can look for specific antibodies to hundreds of substances using only one needle.