An allergy blood test measures a substance called immunoglobulin E (IgE) in your blood. IgE is an antibody that your body makes. If you have allergies, you may have more IgE in your blood than normal.
Allergies are a common, long-term condition that involves your immune system. Your immune system makes antibodies to fight off viruses, bacteria, and other things that can make you sick. With allergies, your immune system treats one or more harmless substances, such as pollen or peanuts, as a threat. To fight the “threat,” your immune system makes IgE antibodies. This is what causes your allergy symptoms.