Origin of the sentinels..

Macrophages are one of the most flexible cells in human body. they are present in almost all of the organs. They can perform duties in the body which are so extreme that they are almost opposite to each other. A macrophage is generally immunological cell which kills the pathogen and generates subsequent immune response for the protection of the host. It can also function as vaccum cleaner to cleanse the tissue of cell debris.  A more calmer form of this cell is a macrophage which is involved in tissue repair and healing. What happens inside the cell at molecular level ? How can a cell achieve such opposite properties?
Interestingly, same nitrogen atom when present in the forms Nitric Oxide, is killer and in the form of arginine is pro repair in nature.
Now, we know that macrophages are flexible and they can perform dual functions. They are present in different tissues, whereby they perform routine functions. The next question is : When the requirement of these cells increases in the tissue ,where are new cells produced? how do they reach the site of requirement?
This question was partially addressed when a paper in 2010 found that a huge reservoir, upto 70% of monocytes are present as reservoir in spleen. During a ischemic (heart attack) they are released from the spleen and reach heart for its repair activities. It was not known before the role os spleen as monocyte reservoir. Similar question could be raised for the tuberculosis. In tuberculosis MTb infects the lung. Macrophages requirement increases in the tissue. Where are these cells produced? Whether these are directly taken from circulation or they are actively formed in bone marrow? these are all open question which need to be addressed.