Final answer:
An antigen-antibody response to medication is known as an allergy, which is an immune reaction to immediate hypersensitivities that can cause symptoms ranging from mild to severe.
Step-by-step explanation:
When a client has an antigen-antibody response to medication, which could be triggered by the drug itself, a preservative, or a metabolite, this reaction is known as an allergy. This type of immune reaction is due to an immediate hypersensitivity where an antibody-mediated response occurs within minutes of exposure to what is usually a harmless substance. Allergies can lead to a range of symptoms from mild to severe, with the most severe reaction being anaphylactic shock which can be potentially life-threatening and requires treatment with epinephrine.