What Are Blood Disorders?

The main components of blood are:

  • Red blood cells (RBCs), which carry oxygen throughout the body
  • White blood cells (WBCs), which fight infection
  • Platelets, which protect against easy bleeding by helping your blood to clot
  • Plasma, the liquid part of blood that carries blood cells, nutrients, wastes, hormones, and many other substances

Abnormalities in any of these components or in related cells or tissues can cause a blood disorder. The disorder and its symptoms depend on precisely what’s gone wrong.

Some blood disorders—including leukemia, lymphoma, myeloma, myelodysplastic syndrome, and myeloproliferative neoplasms—cause unusually low or high levels of blood cells. In some cases the blood cells don’t form or mature as they should and cannot carry out their normal functions.

  • A low level of RBCs is called anemia. A high level is called polycythemia.
  • A low level of WBCs is called leukopenia. A high level is called leukocytosis.
  • A low level of platelets is called thrombocytopenia. A high level is called thrombocytosis.

Other blood disorders have to do with a deficiency of substances the body needs, such as clotting factors in hemophilia, or with the presence of unwanted substances in the blood, such as abnormal proteins in amyloidosis. There are many other specific types of blood disorders.