Final answer:
The initial exposure of an individual to an antigen, which launches an adaptive immune response, is known as primary exposure, setting the stage for the development of immunological memory and sensitization to potential allergens.
Step-by-step explanation:
The correct answer to the question 'The initial exposure of an individual to an antigen is referred to as' is A. Primary exposure. This term is used to describe the primary immune response, which is the reaction of the adaptive immune system to the first contact with an antigen. Primarily, this leads to the activation of immune cells and the production of antibodies specifically tailored to fight the invading pathogen or allergen. This process is critical for developing immunological memory.
It is important to distinguish between primary and secondary immune response; the latter refers to the response of the adaptive immune system to a second or subsequent exposure to an antigen, mediated by memory cells. For example, in Type I hypersensitivities, such as allergies, after the primary exposure has sensitized the individual, any subsequent exposure to the same allergen would trigger a more rapid allergic reaction.