Macrophages are one of many types of white blood cells (leukocytes) present in body tissues. Macrophages are important in immune response and cell stability because they mobilize in cell tissue to attack large foreign particles such as bacteria, yeast, and dead cells. Macrophages are derived from precursor cells called monocytes that first develop in bone marrow. Monocytes enter the blood and travel throughout the body in the circulatory system. When needed, circulatory monocytes move into tissue, where they become macrophages. Here a lung (alveolar) macrophage is seeking foreign bacteria (Escherichia coli) with specialized cell extensions called filopodia. Macrophages engulf and digest foreign materials in a process known as phagocytosis.

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