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.
