Hematology is the branch of medical practice concerned with conditions of the blood. These conditions range from the common iron deficiency anemia, to cancers originating in the blood, to rare forms of abnormal blood coagulation.

The blood as an “organ” literally affects all other organs as it flows through the body and therefore abnormalities in the blood can cause malfunctioning in all other organs.

The blood is composed of particles, cells and sub-cellular elements manufactured in the bone marrow and lymphatic organs. It is also composed of various fluids and proteins. Any of the individual components may be afflicted with abnormalities that require both the diagnostic and therapeutic interventions of a hematologist.

Historically, the first medicines that had any impact on cancer were for leukemia and the lymphatic cancers, Hodgkin's disease and lymphomas. The doctors caring for these patients were primarily hematologists.

Since the early 1950s, the majority of medicines developed to fight cancer have had bone marrow suppression as their major side effect, so doctors dealing with these medicines had to be well versed in treating blood conditions as well.