What type of reaction occurs when the body has an exaggerated response to a substance?

The correct answer is hypersensitivity. This term specifically refers to an exaggerated immune response by the body to a substance that is typically harmless to most individuals. Hypersensitivity reactions can manifest in various forms, including allergic reactions, which may cause symptoms ranging from mild (like rashes) to severe (such as anaphylaxis).

Idiosyncratic reactions, while they do refer to unusual responses to substances, are characterized by unexpected reactions that are not related to the pharmacological actions of the drug; they don’t necessarily imply an exaggerated response.

The term cumulative effect relates to the effects of a drug increasing in intensity when given repeatedly over time, particularly because the drug builds up in the body. This is not about an exaggerated immune response.

Paradoxical reactions are instances where a drug or substance causes effects opposite to what is typically expected, which also does not align with the concept of hypersensitivity.

Hypersensitivity captures the essence of an exaggerated response and is therefore the most accurate choice.