Final answer:
The process by which patients develop a heightened immune response to a substance after initial exposure is called sensitization, and it is characteristic of a Type I hypersensitivity reaction.
Step-by-step explanation:
Patients can develop sensitivity to a substance that did not previously cause a reaction. This process is known as sensitization. Sensitization occurs when an individual's immune system becomes reactive to a foreign substance (an allergen) after initial exposure. The next time the person encounters the same substance, their body has a heightened response. This is the hallmark of a Type I hypersensitivity reaction.
During the first encounter with an allergen, a primary IgE antibody response occurs without apparent symptoms. However, this initial exposure leads to a state of sensitization. Upon subsequent exposures, the allergen can trigger a rapid and often aggressive immune response, as the body has been 'primed' to recognize and fight against the specific substance.
Type I hypersensitivities can range from minor reactions, such as hives and hay fever, to severe reactions like systemic anaphylaxis. Thus, the correct answer to the question is D. sensitization.