[Reverse or inverse reaction - a present-day point].

The reversal or inversal reactions, well studied in some research centers, is still imperfectly known, even contested or denied. This lack of knowledge has been one of the hindrances in the understanding of the mechanism of the hansenic neuritis. The evolution of the concepts about this question is studied, starting with Souza Lima and Souza Campos (1950), followed by Tajiri (1955), up to the present "I reaction" of Jopling, involving the "up grading" and "down grading" reactions. An interpretation of the reactional mechanism is given, with better possibilities for the understanding of clinical aspects and for therapy.