Hematology is the study of blood and the diseases that affect it. Red blood cells, white blood cells, and platelets are the primary cells floating in your blood. Red blood cells carry the oxygen in your blood. White blood cells make up the bulk of your immune system. Platelets help you make clots and stop bleeding. Abnormalities in these cell types can be found on basic lab work, but can be caused by a large number of factors. It is the job of the hematologist to figure out why the abnormality is present.
Hematology also studies the function of the blood. Sometimes people may have a tendency to bleed too easily or develop blood clots too easily. Hematologists fully understand the physiology of bleeding and clotting, and can help identify if there is an underlying disease leading to your symptoms.
Most of hematology is benign, including anemia, thrombocytopenia, myeloproliferative disorders, clots, and bleeding disorders. There is also a component of malignant hematology, which includes leukemia, lymphoma, multiple myeloma, myelodysplasia, and other disorders. These disease and treatments are covered in more detail in the oncology section. The element of malignant hematology is why hematologists are often also trained as oncologists.