Tumors of the hematopoietic and lymphoid tissues

Blood cancer is a type of cancer or tumor. Blood cancer hurts the blood, bone marrow, lymph, and lymphatic system. Sometimes more than one body part is hurt.
The cause is usually different from other types of cancer. The way to find out if a person has this cancer is also different. 
People with blood cancers see a kind of doctor called a hematologist (blood doctor) or oncologist (cancer doctor). There are other blood problems that are not blood cancers. A blood doctor also helps people with those problems.
Medical terms.
Tumors of the hematopoietic (blood cell forming) and lymphoid tissues are local growths. Hematopoietic and lymphoid "malignancies" are cancers that affect the blood, bone marrow, lymph, and lymphatic system. As the blood and lymph connect through both the circulatory system and the immune system, a disease affecting one will often affect the others as well.
Types.
There are three types: 
Finding out.
A doctor will do some tests to find out if a person has a blood cancer. One common test is a complete blood count. The cells that circulate in the bloodstream are generally divided into three types: white blood cells (leukocytes), red blood cells (erythrocytes), and platelets (thrombocytes). The relative proportions is what picks up indications of a problem.
Sometimes the doctor will take a very small piece of the body out, to look at it. This is called a "biopsy". Bone marrow is where new blood is made in the body. A bone marrow biopsy looks at the blood-making parts to see if they have cancer.
Treatment.
People with blood cancer often take drugs called chemotherapy or immunotherapy. If the blood cancer is in one place, they may have radiotherapy. A bone marrow transplant is a difficult way to kill blood cancers.
How many people get it.
Thousands of people get blood cancers. Each year, 30,000 people in the United Kingdom (UK) learn that they have blood cancer.
Lymphomas are more common than leukemias.