Final answer:
Upon a second exposure to an allergen, IgE-primed mast cells bind to the antigen and release chemical mediators like histamine, causing allergy symptoms. The allergic reaction can vary from mild to severe and is quicker and stronger than the first exposure response.
Step-by-step explanation:
During the first exposure to an allergen, the body's immune system reacts by producing antibodies known as immunoglobulin E (IgE). These IgE antibodies bind to mast cells present in the body's tissues. This initial phase is called sensitization and does not usually cause an allergic reaction.
Upon a second exposure to the same allergen, the already primed mast cells with bound IgE quickly recognize the allergen. The binding of the antigen to these IgE-primed mast cells triggers them to release various chemical mediators, such as histamine and cytokines. These substances cause the symptoms of an allergic reaction, ranging from mild, such as sneezing and itchy eyes, to severe reactions like anaphylactic shock, which can potentially be life-threatening. The immediate release of these chemicals leads to various allergy symptoms and can happen within minutes of exposure. Notably, the response is stronger and faster than the primary immune response. In some cases, like with poison ivy, the specific reaction is known as type IV hypersensitivity, where memory T cells activate on re-exposure, leading to an inflammatory response.