Scanning electron micrograph of macrophage on the surface of an endothelium. Macrophages are a type of white blood cell that engulfs and digests cellular debris, foreign substances, microbes, cancer cells, and anything else that does not have the types of proteins specific of healthy body cells on its surface in a process called phagocytosis. These large phagocytes are found in essentially all tissues, where they patrol for potential pathogens by amoeboid movement. They take various forms (with various names) throughout the body, but all are part of the mononuclear phagocyte system. Endothelium is a type of epithelium that lines the interior surface of blood vessels and lymphatic vessels, forming an interface between circulating blood or lymph in the lumen and the rest of the vessel wall. The endothelium is a thin layer of cells that line the lymph and blood vessels, secreting substances that prevent blood clotting, toning vascular smooth muscle, and performing many other complex functions. On the endothelium are macrophages, white blood cells, (leukocytes), or mononuclear phagocytes that ingest bacteria as part of the immune response to infection. Magnification = x3,000.