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Explainer: What Is Hematology?

Patients might see a hematologist if they have a bleeding disorder, anemia, sickle cell disease, leukemia or other conditions related to blood or blood components. Learn more about this medical specialty.

A woman prepares to give a blood sample

The 17th century amateur scientist, Antonie van Leeuwenhoek, had an unusual interest for his time: He wanted to see what the naked eye could not, sights only visible with technological help. In 1674, after designing and building a single-lens microscope, van Leeuwenhoek chose to look at a blood smear. His discovery: red blood cells.

Subsequent scientists would come to share van Leeuwenhoek’s curiosity about blood. A century later, the British anatomist and physiologist William Hewson identified fibrinogen, a protein integral to the process of coagulation and its strength.

These discoveries helped forge a new field of biological study called hematology, or the study of blood. In time, as technology  improved, researchers discovered blood’s complexity and its many biological functions. These are:

  • fighting infection
  • coagulating at the site of an injury, whether at skin level or inside the body
  • preventing life-threatening blood clots in the vascular system
  • using the vascular system—an estimated 60,000 miles of arteries, veins and capillaries— to deliver oxygen and nutrients where they need to go while also and removing carbon dioxide and other waste materials.

As researchers discovered more, they began to identify diseases whose origins could be traced to the blood. These origins were found to be genetic in nature, inherited, or, as in the case of anemia, a disorder that could have multiple causes. They also found that blood can reveal much about its owner's physical health, namely, with biomarkers. Those telltale molecular signs have helped in diagnosing and treating any number of conditions.

Today, a patient might see a hematologist if they have a bleeding disorder, anemia, sickle cell disease, leukemia or another condition related to blood or blood components. Hematology is often mentioned alongside oncology, which is the treatment of cancer, because the two disciplines are related.

“Benign hematology” means conditions other than cancer – an area of study central to the work of CSL Behring and CSL Vifor, two business units of CSL, which make medicines for rare diseases and anemia. In April, CSL Behring will host its Plasma Academy, an educational opportunity for hematology fellows to learn more about benign hematology.

Blood’s components

Blood is comprised of:

  • Plasma
  • Red blood cells, also called erythrocytes
  • White blood cells, or leukocytes
  • Platelets, or thrombocytes

These four components provide vital functions.

Plasma: Plasma is the liquid part of blood and its primary role is transportation. Plasma carries water, salts and proteins. It delivers nutrients and hormones where they need to go.

Red blood cells: The red and white blood cells and platelets comprise the comprise the circulating cells of the blood. Red blood cells make up 99% of the cells. These cells transport oxygen throughout the body and carry carbon dioxide to the lungs to be exhaled. It is the hundreds of millions of hemoglobin molecules inside each red cell that carries the oxygen.  A hormone called erythropoietin, made mostly in the kidneys, controls production of red blood cells. Most blood cell production happens in bone marrow.  

White blood cells: White blood cells belong to the body’s complex  immune system, which also includes tonsils, bone marrow, lymphatic system and more. Like an army, the immune system defends the body from outside invaders, like viruses and bacteria. Various types of white blood cells, which have different shapes and functions, operate sort of like platoons. Neutrophils, the first line of defense against an invader, are the most plentiful. T cells help control how other immune cells respond to invaders and attack already infected cells. B cells make antibodies, custom-designed cells that attack specific invaders. 

Platelets: Platelets are like tiny corks that plug up the hole in a blood vessel, an important first step in coagulation. Clotting factor proteins, of which there are 12, provide a kind of cement to strengthen the plug. 

Blood disorders: Inherited blood disorders, passed from parent to child, are considered rare diseases. The U.S. Centers for Disease Control and Prevention (CDC) says 33,000 people in the United States have hemophilia, out of a population of 350 million people. The most common type of heritable blood disorder, von Willebrand disease, affects 1% of the U.S. population. People with blood disorders bruise easily, their bleeding can be tough to stop and it can sometimes happen spontaneously without an obvious injury.

Hemophilia. If someone has hemophilia, their blood lacks one of the clotting factors it needs or the clotting factor they have doesn’t work as it should. Most patients are men, and most of them inherited the disease from their mothers. People with hemophilia A lack Factor VIII. Patients who have hemophilia B lack Factor IX. Problems include prolonged bleeding after an invasive procedure, like tooth extraction, and risk of serious complications if a bleed reaches internal organs or muscle. But with treatment to replace the missing clotting factor – or with gene therapy – people who have hemophilia can live full, healthy lives.

Von Willebrand disease. When an injury occurs and platelets arrive to stanch the blood flow, platelets begin to collect some clotting protein to cement them in place, which is what von Willebrand factor does. When working normally, this protein emerges from the lining of the injured blood vessel to help with the clotting and bind with Factor VIII (the factor deficient in Hemophilia A). When there isn’t enough von Willebrand factor, or it’s malfunctioning, the blood clotting process is too slow, causing problems like heavy periods for women. Men, in equal numbers, also have VWD, which has three types and even subtypes, depending on which part of the von Willebrand protein isn’t working properly. As with hemophilia, the more severely affected patients receive factor replacement therapy to correct the deficiency.

Learn more about factor deficiencies on

Visit the National Bleeding Disorders Foundation and the World Federation of Hemophilia to learn more about advocacy for patients.