Macrophage and Borrelia bacteria, illustration. Macrophages are cells of the body's immune system. They phagocytose and destroy pathogens, dead cells and cellular debris. Here, the macrophage (pink) is engulfing the bacteria (dark blue), a process known as phagocytosis. The bacteria will then be destroyed. Several diseases are caused by various types of Borrelia bacteria, including Lyme disease and relapsing fever.