Macrophages can be found throughout the body. They can be considered the sentinel cells of the immune system. They are often the first to recognize infection and respond by mobilizing other cells of the immune system. Normally, macrophages are in a resting state, and carry out a low level of phagocytosis as needed. In this case, antibody and complement are called opsonins , molecules that enhance phagocytosis.
Opsonins activate macrophages to increase phagocytosis and to release cytokines.
During ADCC, cells are killed by direct release of toxic proteins, hydrolytic enzymes, and reactive oxygen intermediates onto their membranes. The mediators of antimicrobial and cytotoxic activity of macrophages are listed in table 2-5 . Note that neutrophils use many of the same killing mechanisms as macrophages. Macrophages serve as antigen presenting cells to T helper cells. Macrophages express abundant amounts of MHC class II upon which antigens are displayed for activation of T helper cells.