Final answer:
Initial exposure to an allergen does not always cause an allergic reaction but primes the immune system with IgE antibodies. Subsequent exposures can then trigger the noticeable allergic reaction.
Step-by-step explanation:
Does initial exposure to an allergen always cause an allergic reaction? Not necessarily. On initial exposure to an allergen, the immune system may not manifest a reaction; however, it synthesizes antibodies of the IgE class that prime the individual for a potential allergic reaction upon subsequent exposures. This priming phase is an essential part of developing a type I hypersensitivity. During this phase, the allergen-specific IgE antibodies bind to mast cells, sensitizing the individual without causing an external allergic response at that moment. On secondary exposure to the same allergen, these primed mast cells are activated by allergen-IgE binding, which in turn release chemical mediators, including histamine and serotonin, leading to an allergic reaction.