An allergic response is an immune system reaction to a substance that is typically harmless, such as pollen or certain foods. When someone with an allergy encounters this substance, their body produces antibodies called Immunoglobulin E (IgE). The next time they come into contact with the same substance, these IgE antibodies recognize it and trigger a release of chemicals such as histamine. This leads to symptoms like sneezing, runny nose, or hives - an allergic reaction.