Each of the cells and platelets found in the blood arise through a process called hematopoiesis. Hematopoiesis begins with a single type of stem cell. These stem cells, known as hemocytoblasts and are located in red bone marrow in children and adults. In a developing fetus, hemocytoblasts are also found in the liver, spleen and lymph nodes as well as bone marrow. Ultimately, through the process of hematopoiesis, hemocytoblasts will differentiate into red blood cells, white blood cells, and cell fragments known as platelets. As we will learn later, platelets are important in blood clotting processes. White blood cells are discussed later in module 3 as well as in the lab portion of this course, but it would be good to remember that macrophages are large phagocytic cells that gobble up foreign material and damaged cell parts. Macrophages perform an important step in the breakdown of red blood cells. Neutrophils are very common with infections and Lymphocytes are an important part of our immune system and are the source of antibodies which you have probably heard about. We will talk about lymphocytes later.