When an allergic person comes into contact with an allergen, their immune system recognizes it as a foreign invader and initiates an immune response. The immune systems of people without allergies still recognize these substances, but usually ignore them. The scientific process behind what makes allergies, allergies, has a couple different steps:

First, your body needs to come in contact with a substance before developing an allergy. This initial exposure usually does not produce an allergic response. It is later exposures that do— sometimes the exposure directly after, and sometimes many exposures later.  This is why you may hear someone say, “I have eaten shrimp for 20 years without a problem, and now I’m allergic!”
Upon exposure to an allergen, the allergic person’s immune system reacts by producing specific antibodies (called immunoglobulin E, or IgE).

Next, these IgE antibodies bind to specific cells (called mast cells and basophils). These specific cells are mostly found in the respiratory tract, the skin, and the gastrointestinal tract. 

Finally, when the allergen interacts with the IgE antibodies bound to these cells, this interaction triggers the body’s inflammatory response, causing the release of chemicals like histamine. The release of these chemicals is what causes the characteristic symptoms of an allergic reaction, which can vary from itching and sneezing to rashes and hives.

Below, we dive deeper into what everyone would like to know: “Why do I have them?”