When monocytes enter the tissues and become macrophages they undergo several changes. The cells enlarge and increase the amount of intracellular lysosyme allowing greater phagocytosis. In the tissues, macrophages live for months or years and may be motile. Macrophages move with amoeboid movements using pseudopods.
Macrophages are usually in the resting state unless activated during an immune response.
Phagocytosis of antigens may also stimulate activation. Activated macrophages have an important role in phagocytosis. In this role they recognize and remove unwanted particulate matter including products of inflammation and invading organisms, antigens and toxins. Finally, macrophages have an important secretory role. In chronic inflammation, macrophages act as scavengers and also become giant cells in the formation of granulomas.