Figure 1.

Development of tissue resident macrophages: Tissue resident macrophages are derived from either blood monocytes or fetal yolk sac. Depending upon the signals they receive, they form tissue specific macrophages. PU.1 and macrophage colony stimulating factor are required for the differentiation into tissue resident macrophages. These tissue resident macrophages can be polarized into either M1 or M2 macrophages. M1 macrophages are activated macrophages which result in a Th-1 response (killing intracellular pathogen) whereas M2 macrophage are alternatively activated macrophages which result in a Th-2 response (wound healing, and tissue repair).