Allergy is an overreaction of your immune system in response to something that is normally harmless. In an allergic reaction to substances such as pollen, house dust mites, certain foods or insect stings, the immune system produces antibodies called Immunoglobulin E (IgE) specific to the allergen it’s trying to fight. The process of your body becoming sensitive to an allergen is called sensitisation.
When you have an allergic reaction, the IgE antibodies tell other cells to release certain chemicals, such as histamine. And it’s those chemicals that can cause the annoying and sometimes even life-threatening allergy symptoms. The difference between sensitisation and allergy is whether your immune system uses its defence mechanisms or not. Meaning whether the chemicals that cause allergy symptoms are released or not. If you’re allergic, your next encounter with the allergen might trigger an allergic reaction. But you can be sensitised and encounter the specific substance repeatedly and never experience allergy symptoms. Sensitisation does not always lead to symptoms, but symptoms do not develop without sensitisation.