Final answer:
During initial exposure to an allergen, the main event is the sensitization of the immune system where B cells produce IgE antibodies that bind to mast cells, preparing for a possible allergic reaction upon subsequent exposure.
Step-by-step explanation:
Initial Exposure to Allergens and Immune Response
When a genetically susceptible person is initially exposed to an allergen, the immune system's response is not an allergic reaction but rather a sensitization process. The first event that occurs is that B cells are activated by TH2 cells through cytokines such as interleukin-4 (IL-4) and interleukin-13 (IL-13). These B cells then proliferate and differentiate into plasma cells that produce IgE antibodies. IgE, once synthesized, binds to the surface receptors of mast cells without causing an immediate allergic reaction. This process primes the mast cells so that upon subsequent exposure to the same allergen, the individual will have an allergic reaction as the allergen binds to the IgE antibodies on mast cells, leading to the release of histamines and other chemical mediators that cause allergy symptoms.