There are five types of leukocytes (white blood cells) found in the blood:
Basophils
Eosinophils
Lymphocytes
Monocytes
Neutrophils
Eosinophils
Lymphocytes
Monocytes
Neutrophils
Each cell type has a specific role to play in our body's immune system. Important overall functions of the immune system include defense against invading microorganisms, removal of dead or damaged cells, and destruction of cancer cells.
Blood cell formation begins with a special cell located in the bone marrow called a hematopoietic stem cell. Unlike most cells, the hematopoietic stem cell has the capability for self-renewal as well as the ability to give rise to any one of the separate blood cell types. This means that as the stem cell divides in two, one cell replaces the original stem cell and the other begins the multi-step process of developing into a mature blood cell.
It is important to note that hematopoietic stem cells are different from embryonic stem cells. While hematopoietic stem cells can develop into any type of blood cell, embryonic stem cells can develop into any cell type in the body.
Blood cell precursors usually progress through a series of stages in the bone marrow before entering the circulating blood stream. Signals from surrounding cells in the bone marrow can help stimulate stem cells to divide as well as develop and mature into specific blood cell types. Normal cell division is also regulated by an important process known as the cell cycle. Disruption of this process is central to the development of leukemia.
