Immunity and immune reactions are facilitated mostly through the actions and secretions of leucocytes, both granular and agranular. Monocytes and neutrophils are phagocytic cells that engulf damaged body cells and foreign matter. When a monocyte leaves the bloodstream and enters the tissue, it is called a wandering macrophage. Monocytes and macrophages often participate in antigen recognition, making the initial contact with the antigen and presenting it to other leucocytes, such as lymphocytes, in order to trigger a full immune reaction. This reaction involves the rapid proliferation of leucocyte subpopulations.