Sensitisation is the first step of developing allergy. It means that your body has become sensitive to an allergen and has created antibodies called Immunoglobulin E (IgE) as part of the defence against it.
Allergic reactions don’t happen the first time you encounter an allergen. Your body needs to prep its defence mechanisms first. The immune system has to recognise and memorise the allergen before it can produce specific IgE antibodies. The process of your body becoming sensitive to an allergen is called sensitisation.