This lecture covers the activation of macrophages by Th1 cells, focusing on the immune response to Listeria infection. It explains how activated macrophages kill Listeria in vitro and the production of cytokines by Th1 cells. The lecture also discusses diseases like tuberculosis and helminth infections, highlighting the role of Th1 and Th2 responses. Additionally, it explores the modulation of B cells, hematopoiesis, and the immune response against M. tuberculosis and helminthes. The importance of Th17 cells in mucosal protection and the role of Treg cells in tolerance are also addressed.