Allergies and the Immune System 

An allergic response occurs when your immune system makes antibodies to something that is otherwise harmless. Antibodies are proteins made by cells of the immune system to fight off substances which the body marks as foreign. Typically, we think of antibodies as helping us to fight off pathogens, such as bacteria or viruses, but sometimes antibodies are made for allergens like pollen, pet dander, or mold, which leads to allergies.  

Allergic reactions occur when you are exposed to an allergen. In response, special cells of your immune system, called B cells make an antibody called IgE. This antibody responds to the allergen and leads to an allergic reaction. This is the most common type of allergic reaction that people think of and is called type one hypersensitivity. 

You usually have to be exposed to an allergen more than once before you have an allergic reaction. This is because the first time the immune system sees something, it’s considered priming, or primary exposure. During primary exposure, your immune system is checking out the allergen, learning how to respond, and preparing immune cells with IgE to respond in the future.  Then when exposed to an allergen in the future, your immune cells will already have the instructions from your first exposure, which allows your immune response and therefore allergic reaction to be more powerful the second time you encounter an allergen.