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When you have a blood test to query response to an immunization, this is also IgG testing. A common example is a “Rubella titer.”
In the context of food, IgG signifies memory through exposure to a food. Because a normal immune system should make IgG antibodies to foreign proteins (to include foods), a positive IgG test to a food is a sign of a normal immune system, and suggests tolerance or “memory” of the food rather than food allergy. Therefore, IgG testing is not recommended for evaluation of food allergies.
If the patient has previously eaten the food (milks, eggs), he or she would likely have IgG to the food.