Most of the time, blood functions normally, but sometimes, blood disorders or diseases can cause problems. Diseases of the blood that commonly affect people can involve any or all of the three types of blood cells (red blood cells, white blood cells, or platelets) or the proteins and chemicals in the plasma that are responsible for clotting.

Some of the diseases and conditions involving the blood include:

Diseases of the Red Blood Cells
The most common condition affecting the red blood cells of teens is anemia (pronounced: uh-nee-mee-uh), a lower-than-normal number of red cells in the blood (RBC). Anemia is accompanied by a decrease in the amount of hemoglobin present in the blood. Anemia symptoms — such as pale skin, weakness, and a fast heart rate — happen because of the blood's reduced capacity for carrying oxygen. Causes of anemia can be grouped into two categories: anemia caused by insufficient RBC production and anemia caused by RBCs being destroyed too soon.

Anemia resulting from inadequate red blood cell production:
Several conditions can cause a reduced production of red blood cells, including:
Iron deficiency anemia. Iron deficiency anemia is the most common type of anemia and can affect people who have a diet low in iron or who've lost a lot of RBCs (and the iron they contain) through bleeding. Premature babies, infants with poor nutrition, menstruating teenage girls, and those with ongoing blood loss due to illnesses such as inflammatory bowel disease are especially likely to have iron deficiency anemia.
Anemia due to chronic disease. People with chronic diseases (such as cancer or human immunodeficiency virus infection [HIV]) often develop anemia as a complication of their illness.
Anemia due to kidney disease. The kidneys produce erythropoietin, a hormone that stimulates production of red cells in the bone marrow. Kidney disease can interfere with the production of this hormone.
Anemia resulting from unusually rapid red blood cell destruction:
When red blood cells are destroyed more quickly than normal by disease (this process is called hemolysis, pronounced: hih-mah-luh-sus), the bone marrow will make up for it by increasing production of new red cells to take their place. But if RBCs are destroyed faster than they can be replaced, a person will develop anemia. Several causes of increased red blood cell destruction can affect teens:
G6PD deficiency. G6PD is an enzyme that helps to protect RBCs from the destructive effects of certain chemicals found in foods and medications. When the enzyme is deficient, these chemicals can cause red cells to hemolyze, or burst. G6PD deficiency is a common hereditary disease among people of African, Mediterranean, and Southeast Asian descent.
Hereditary spherocytosis (pronounced: sfeer-o-sye-toe-sus). In this inherited condition, RBCs are misshapen (like tiny spheres, instead of disks) and especially fragile because of a genetic problem with a protein in the structure of the red blood cell. This fragility causes the cells to be easily destroyed.
Autoimmune hemolytic anemia. Sometimes — because of disease or for no known reason — the body's immune system mistakenly attacks and destroys red blood cells.
Sickle cell anemia. Most common in people of African descent, sickle cell anemia is a hereditary disease that results in the production of abnormal hemoglobin. The RBCs become sickle shaped, cannot carry oxygen adequately, and are easily destroyed. The sickle-shaped cells also tend to abnormally stick together, causing obstruction of blood vessels. This blockage in the blood vessels can seriously damage organs and cause bouts of severe pain.

Diseases of the White Blood Cells
Neutropenia (pronounced: noo-truh-pee-nee-uh) occurs when there aren't enough of a certain type of white blood cell to protect the body against bacterial infections. People who take certain chemotherapy drugs to treat cancer may develop neutropenia.
Human immunodeficiency virus (HIV) attacks certain types of white blood cells (lymphocytes) that work to fight infection. Infection with the virus can result in AIDS (acquired immunodeficiency syndrome), leaving the body prone to infections and certain other diseases. Teens and adults can get the disease from sexual intercourse with an infected person or from sharing contaminated needles used for injecting drugs or tattoo ink.
Leukemias (pronounced: loo-kee-mee-uhz) are cancers of the cells that produce white blood cells. These cancers include acute myeloid leukemia (AML), chronic myeloid leukemia (CML), acute lymphocytic leukemia (ALL), and chronic lymphocytic leukemia (CLL). The most common types of leukemia affecting kids are ALL and AML. Scientists have made great advances in treating several types of childhood leukemia, most notably certain types of ALL.
Diseases of Platelets
Thrombocytopenia (pronounced: throm-buh-syte-uh-pee-nee-uh), or a lower than normal number of platelets, is usually diagnosed because a person has abnormal bruising or bleeding. Thrombocytopenia can happen when someone takes certain drugs or develops infections or leukemia or when the body uses up too many platelets. Idiopathic thrombocytopenic purpura (ITP) is a condition in which the immune system attacks and destroys platelets.
Diseases of the Clotting System
The body's clotting system depends on platelets as well as many clotting factors and other blood components. If a hereditary defect affects any of these components, a person can have a bleeding disorder. Common bleeding disorders include:

Hemophilia (pronounced: hee-muh-fil-ee-uh), an inherited condition that almost exclusively affects guys, involves a lack of particular clotting factors in the blood. People with severe hemophilia are at risk for excessive bleeding and bruising after dental work, surgery, and trauma. They may experience episodes of life-threatening internal bleeding, even if they haven't been injured.
Von Willebrand disease, the most common hereditary bleeding disorder, also involves a clotting-factor deficiency. It affects both guys and girls.

Other causes of clotting problems include chronic liver disease (clotting factors are produced in the liver) and vitamin K deficiency (the vitamin is necessary for the production of certain clotting factors