How do blood cells normally develop?

About 95% of the blood cells are formed in the bone marrow through a process known as hematopoiesis. In the bone marrow, all blood cells start out as stem cells, which are referred to as the first phase of blood cells.

As the young, immature stem cells mature, they evolve and differentiate into several distinct types of cells, including erythrocytes (red blood cells), leukocytes (white blood cells), and platelets. Some immature blood cells, known as blasts, stay and mature in the bone marrow while others travel to other parts of the body where they develop into mature, functional blood cells. The rate at which blood cells are produced is regulated by the body's needs.

Some organs and systems in the body, such as the liver, lymph nodes, and spleen, play a role in regulating the production, differentiation, and destruction of blood.