Blood is composed of four components: white blood cells, red blood cells, platelets and plasma. Each of these components possesses their own specific function to transport oxygen, fight disease, increase hemoglobin levels, and more. When one may have a deficiency or exceeds normal levels of any of these components, an abnormal blood test result will occur. A hematologist specializes in the diagnosis, treatment, and prevention of disorders and cancers that affect the blood, bone marrow and the immunologic, hemostatic and vascular systems.