Reaction: "A response to a substance, treatment, or other stimulus, such as an antigen-antibody reaction, an allergic reaction, or an adverse pharmacological reaction." - Mosby's Medical Dictionary This class documents an Observation of an adverse physiological response attributed to the exposure of the Patient to the given substance. Note that the dates may be unspecified, such as when the Patient reports that she breaks out in hives when she eats peanuts. "Details about each adverse reaction event linked to exposure to the identified substance." - HL7 FHIR, AllergyIntolerance.reaction