Final answer:
Type I hypersensitivities begin with sensitization, where the immune system first encounters an allergen and produces IgE antibodies. These antibodies bind to immune cells and, upon re-exposure to the allergen, result in an allergic response. Sensitization is distinct from opsonization, phagocytosis, and complement activation.
Step-by-step explanation:
Type I hypersensitivities, also known as immediate hypersensitivities, such as an allergy to pollen, require an initial priming event known as sensitization. During sensitization, the immune system is first exposed to the allergen. The body mistakes it for a harmful substance and produces immunoglobulin E (IgE) antibodies specific to the allergen. These IgE antibodies bind to mast cells and basophils, and upon subsequent exposure to the same allergen, they trigger these cells to release histamine and other inflammatory mediators, leading to allergic symptoms.
It is important not to confuse this process with opsonization, phagocytosis, or complement activation, which are different immune mechanisms. Complement activation can indeed be a consequence of the antibody response, but it is not the priming event for Type I hypersensitivities. Opsonization and phagocytosis are part of the immune system's way of eliminating pathogens, typically after they have been marked by antibodies or complement proteins.