Adverse reactions to immunotherapy are unique compared to other treatments in that they can occur in any tissue and organ of the body, mostly within 4 weeks of treatment initiation, but can also occur at any time after treatment initiation, including after treatment has stopped.
Based on the average time to onset of adverse reactions, they can be broadly divided into early toxicity (within 2 months of treatment initiation) and delayed toxicity (beyond 2 months). The former includes skin toxicity (e.g., rash, pruritus, vitiligo), gastrointestinal toxicity (e.g., diarrhea, colitis), and hepatotoxicity (e.g., hepatitis); the latter includes pneumonia, endocrine toxicity (e.g., hyperthyroidism and hypothyroidism), and neurotoxicity (e.g., encephalitis).
Of these, rash and diarrhea are more common, occurring in more than 10% of patients, while adverse reactions such as encephalitis and pneumonia are less common, occurring in no more than 3% of patients.