Macrophage. Illustration of an activated macrophage cell. In its active state the cell surface appears frilly and these frills" spread out as arms to draw the cell along the substrate as it seeks and engulfs particles. Wandering macrophages are monocyte white blood cells from circulating blood that function to defend the body against invasion by foreign organisms. Macrophages migrate to tissues such as the lymphatic system and lungs; some types of macrophage migrate to sites of infection. Macrophages can also detect antigenic (foreign) substances during the initial stage of the body's immune response."