Monocytes circulate in the blood for 1-3 days. Once in extravascular tissues they have a life span of several months, and they can divide and multiply locally within the tissues to maintain their tissue populations. The major inflammatory function of macrophages was earlier thought to be phagocytosis. Macrophages are referred to as the second line of cellular defense. They phagocytose the offending agent as well as the resultant cellular debris.
Macrophages are more efficient and multi-purposed than neutrophils.
However, more recently, secretory functions of macrophages have been elucidated. They are extremely important for antigen processing and presentation to lymphocytes. Many monokines have immunoregulatory functions. Macrophages can be cytotoxic and participate in antitumor and antiviral mechanisms. Mononuclear phagocytes are a prominent component of chronic inflammation.