A type 1 reaction (T1R) is also known as a reversal reaction. This phenomenon involves exacerbation of the immune system or delayed-type hypersensitivity in response to the antigens of Mycobacterium leprae present in parasitized tissues. It occurs in most patients of the tuberculoid and borderline forms of the Ridley & Jopling classification for leprosy. It is an important phenomenon that can occur before, during, or after leprosy treatment and can be destructive, causing tissue damage mainly in the nerves, as well as irreversible sequelae. The recognition of T1R in histological sections may be notified prior to clinical presentation. Histopathological recognition is vital in defining or confirming the presence of T1R, guiding the treatment of the reaction process, avoiding or reducing the possibility of serious sequelae, correcting possible mistakes in the classification of patients within the spectrum of leprosy, and ruling out other diseases that can clinically simulate a T1R. In this chapter, the histopathological characteristics that allow the recognition of T1R, various histopathological aspects of the common forms of leprosy, and histopathological differential diagnoses are discussed.
Keywords: Downgrading, Hansen’s disease, Leprosy, Reaction type 1, Reversal reaction, Upgrading.