[Defense mechanisms against bacteria of intracellular development]

Intracellular bacteria are not inhibited by antibodies. Therefore the main mechanisms of resistance against these pathogens are the influx and activation of mononuclear phagocytes by the synergistic interaction of gamma-interferon and tumour necrosis factor. During the early phase of infection, gamma-interferon is produced by IL-12-activated natural killer cells (IL-12 being mainly produced by macrophages). In the later phase of infection acquired immunity is T-cell-mediated and involves the Th1 CD4+ subpopulation. These cells produce gamma-interferon when triggered by antigen. There is a complex network of interactions among cytokines. Some cytokines act synergistically to enhance resistance to infection, yet antagonistic interactions can sometimes occur.