Final answer:
To be allergic to something, you do usually need prior exposure, during which sensitization occurs. Following this, the immune system can rapidly react to the allergen upon re-exposure, resulting in an allergic reaction classified as a Type I Hypersensitivity.
Step-by-step explanation:
An allergy occurs when an individual's immune system reacts adversely to a normally harmless substance in the environment. For most allergic reactions to take place, prior exposure to the allergen is necessary. This prior exposure leads to sensitization, where the immune system generates a memory response to the allergen. On subsequent contact with the allergen, the sensitized individual's immune system rapidly recognizes the substance, resulting in an allergic reaction.
These reactions are classified as Type I Hypersensitivities, which are immediate in nature. For instance, an allergy to pollen is considered an immediate hypersensitivity (Type I), which is triggered when a presensitized individual is re-exposed to the allergen, causing symptoms such as watery, itchy eyes, and sneezing.
Sensitization can also happen with contact allergens such as poison ivy, where the first contact does not result in a reaction, but subsequent exposures activate memory T cells, causing a type IV hypersensitivity reaction known as contact dermatitis.