Macrophages
Macrophages are cells produced by the differentiation of monocytes in tissues. Macrophages function in both non-specific defense (innate immunity) as well as help initiate specific defense mechanisms (adaptive immunity) of vertebrate animals. Though macrophages are activated by a variety of stimuli in the course of an immune response, phagocytosis of particulate antigens serve as an initial activating stimulus. T helper cells are responsible for the activation of macrophages. TH1 cells activate macrophages by signaling with IFN-gamma and displaying the protein CD40 ligand. Activated macrophages are more effective than the resting ones in eliminating potential pathogens, because they exhibit greater phagocytic activity. Activated macrophages secret various cytotoxic proteins that help them eliminate a broad range of pathogens including virus infected cells, tumor cells, and intracellular bacteria. Macrophages are vital to the regulation of immune responses and the development of inflammation; they produce a wide array of powerful chemical substances (monokines) including enzymes, complement proteins, and regulatory factors such as interleukin-1.