What describes allergic reactions to medications or substances?

Allergic reactions to medications or substances are characterized by adverse immune responses. When the body encounters a substance it perceives as harmful, it may launch an immune response that can range from mild symptoms, such as a rash, to severe reactions like anaphylaxis. This immune response involves the production of antibodies and the release of chemicals like histamine, leading to symptoms that can affect various systems in the body.

The other options describe different situations. Gastrointestinal disturbances can be side effects of medications but are not necessarily allergic reactions; they often result from the medication's pharmacological effects rather than an immune response. Common side effects can occur with many medications and do not specifically indicate an allergic reaction, as they may not involve the immune system at all. Antiviral interactions refer to how certain medications affect the efficacy of antiviral drugs, which again is unrelated to the immune response characteristic of allergic reactions.