Introduction
Macrophages were first described as phagocytes in 1882 by Élie Metchnikoff, this cell type has been found residing in almost every tissue of the body as large, tissue-resident myeloid cells, characterized as having pseudopodia and phagocytic granules and by distinct functional profiles. As a central part of the innate immune system, they serve a crucial host defense function, but also contribute to the maintenance of tissue homeostasis by clearing apoptotic and damaged cells. Macrophages also play an essential role during organogenesis in embryonic development, where they are highly concentrated at sites of high cell death, such as the developing limb buds [1, 2]. These tissue re-modeling functions are maintained in adults and support wound healing and tissue repair/remodeling processes after infection and injury. Macrophages can also acquire tissue-specific phenotypes and functions in different organs. Although they exert tissue-specific functions, all such tissue-specific macrophages also release common soluble mediators including enzymes, cytokines, chemokines, and arachidonic acid derivatives, as well as glycoproteins such as fibronectin, that help with the maintenance of homeostasis and tissue repair [3, 4].