These cells were first recognized by Elia Metchnikoff in 1882 in the larvae of starfish upon insertion of thorns of tangerine tree and later in Daphnia magna or common water flea infected with fungal spores as cells responsible for the process of phagocytosis of foreign particles. He described macrophages as a large cell with marked phagocytic ability found in most connective tissues and body fluids throughout the animal kingdom.
Many tissue macrophages are prenatally established during embryonic development from progenitors derived from the yolk sac or fetal liver and in the steady state they are maintained independently from the bone marrow-derived monocytes.
Macrophages reside in every tissue of the body and exhibit a great functional diversity. Depending on the tissue in which they reside, macrophages possess different shapes, e.g., Kupffer cells in the liver or Langerhans cells in the skin. In mice, macrophages present in many tissues, including skin, liver, kidney, and brain, originate from the yolk sac or the fetal liver.
As one of the most effective cell types in the emergency response system of the human body, macrophages modulate host defense, inflammatory processes, and homeostasis.
With powerful abilities of cell engulfment, antigen presentation and cytokine secretion, macrophages govern the initiation and resolution stages of innate and adaptive immunity.
Macrophages are equipped with a broad-range of pattern-recognition receptors (PRRs), which are required for the production of an array of inflammatory and immunosuppressive cytokines, and the uptake of cellular debris and pathogenic material.
The macrophages
