Hypersensitivity reactions are presumably the most frequent immunotoxic effects of medicines and chemicals. However, the understanding of underlying mechanisms, epidemiological data, methods to predict this risk in nonclinical studies are far less extensive than is available as regards immunosuppression.

Hypersensitivity is the prefered term nowadays as "allergy" has been misleadingly used. Hypersensitivity reactions can be induced by an antigen-specific mechanism (immune-mediated or immunoallergic hypersensitivity) or a nonantigen-specific mechanisms (nonimmune-mediated or pseudoallergic hypersensitivity).