Macrophages and dendritic cells occupy a unique niche in the immune system. These cells can participate in host defense to infectious microorganisms, and promote host inflammatory responses. They can also mitigate inflammatory responses and down regulate adaptive immunity. A variety of approaches in Immunology and Cell Biology are being taken to understand the molecular mechanisms by which these fascinating cells can mediate these divergent processes. Studies on host defense to infectious diseases focus on: I) the cell and molecular biology of macrophage infection by intracellular pathogens. II) the molecules produced by macrophages that mediate the killing of intracellular pathogens. III) an analysis of monocyte or macrophage subsets during infectious diseases. Studies relating to the host inflammatory response focus on: I) the regulation of cytokine gene expression in macrophages and dendritic cells. II) receptor-mediated modulation of cytokine production by regulatory macrophages. III) the development of novel anti-inflammatory therapeutics to treat autoimmune diseases.