What event occurs with the sensitizing dose of allergen?

In the context of allergic responses, the sensitizing dose of an allergen is critical for the development of an allergy. When a person is first exposed to an allergen, their immune system reacts by producing specific immunoglobulin E (IgE) antibodies. These antibodies bind to allergens and sensitize the immune system, preparing it for future exposures.

During this initial sensitization phase, the immune system learns to recognize the allergen as a potential threat, leading to the production of IgE antibodies. These antibodies subsequently attach to mast cells and basophils, which are key players in the allergic response. Upon re-exposure to the same allergen, these cells become activated, resulting in the release of histamines and other inflammatory mediators, which contribute to the symptoms of an allergic reaction.

The other choices represent later stages or situations that occur after the sensitization phase. For instance, an immediate allergic response happens upon subsequent exposures when the IgE antibodies trigger mast cells, while memory cells play a role in long-term immunity rather than the initial sensitization. Systemic symptoms develop during acute allergic reactions that typically follow sensitization when the immune system has already "learned" to respond to the allergen. Thus, the correct answer underscores the pivotal