The network of phagocytic cells, such as macrophages and their precursors (monocytes), distributed in the body’s tissues, particularly the bone marrow, liver, spleen, and lymph nodes. Some monocytes circulate in the bloodstream, along with dendritic cells, which also are found in a range of tissues, including the lungs, lymphatic system, digestive system, and skin. Monocytes that enter tissues develop into macrophages, some of which are specialized as histiocytes or Kupffer cells. All have the common purpose of engulfing and destroying microorganisms and other foreign particles as part of the body’s innate immune response. They also serve as antigen-presenting cells to stimulate further more specific immune responses by lymphocytes.