When can delayed hypersensitivity reactions (HSR) typically occur?

Delayed hypersensitivity reactions, also known as Type IV hypersensitivity reactions, typically arise several days to weeks after exposure to an antigen. This type of reaction is mediated by T lymphocytes and does not involve antibodies, which is why the response is not immediate. Instead, the sensitization process occurs upon the first exposure, and subsequent exposures can trigger a more robust immune response.

This timeframe differentiates it from immediate hypersensitivity reactions, which occur within minutes to hours after exposure. Understanding that delayed hypersensitivity is characterized by this extended reaction window is crucial in clinical settings, particularly in monitoring patients for adverse reactions to medications, especially chemotherapy and immunotherapy. Recognizing that these reactions can manifest days to weeks later helps healthcare professionals in appropriately managing patient care and follow-up.