Bone marrow. Computer artwork showing blood cells in the honey-combed interior of bone where they are formed. The interior is cancellous, or spongy, bone. The cells seen here include a red blood cell (RBC, red, far right), a platelet (triangular, lower left) and various white blood cells (WBC, large, round), including macrophages. Macrophages, like the one at lower right, defend the body by engulfing and digesting foreign objects. RBCs are continuously produced as they have a short working life carrying oxygen around the body. WBCs are part of the immune system. Platelets help in the formation of blood clots.