An allergic reaction is an immune system response to a substance that is typically harmless, such as pollen or certain foods. When someone with an allergy encounters this substance (known as an allergen), their body produces antibodies called Immunoglobulin E (IgE). The next time they come into contact with the same allergen, it triggers a release of chemicals such as histamine, which causes symptoms like sneezing, runny nose, and redness or swelling. In severe cases, an allergic reaction can be life-threatening (anaphylaxis), causing difficulty breathing, rapid heart rate, and a drop in blood pressure.