White blood cells are given this name because they appear as fuzzy grayish white layer that is separating the red blood cells from the clear yellow plasma inside a centrifuged tube of human blood.
The white blood cells are also known as “Leukocytes” which literally means “white cells” where the Greek word Leukos means “white”.
The immune system in the human body generates Leukocytes to defend the body from the invasion of foreign microorganisms such like bacteria. For this job to be done effectively, different types of white blood cells work together against the invading foreign microorganisms.
The different types of white cells fight different kinds of infection, have different shapes, and develop in different life cycles.
Blood cells are believed to be differentiated from the Hemocytoblast stem cell. Stem cells are cells that can be differentiated (converted) to other types of cell. Stem cells are found in stem cell pools (compartments) of the Bone Marrow.
Stem cells have different levels of Potency (capability of being differentiated into various kinds of other cells). The Hemocytoblast is a Multipotential stem cell, this means it is capable of differentiating into various types of blood cells, and only blood cell, but it cannot be differentiated into other types of cells that are not blood cells.
To form leukocytes, the Hemocytoblast needs to be differentiated into a Unitpotential stem cell (a stem cell that can be differentiated into one type of cells only). The newly generated cells can be either mature or primitive cells. Primitive cells can be developed later into a mature cell.
There are two major types of White Blood Cells that can be easily recognized under the microscope:
There are 3 types of Granulocytes:
Granulocytes are generated by differentiating the Hemocytoblast into a unipotential stem cell known as the Myeloblast. The Myeloblast then follows a series of differentiation into immature Myelocytes (Marrow Cells). Myelocytes are then developed into the three different types of the mature Granulocytes.
The very final stage in generating Granulocytes happens when the fully mature cells are developed from their earlier life “Band” cells. Band Granulocytes are given this name because their horseshoe shaped nucleus look like bands at this stage. Then the mature cells are called “Segmental” to not be confuse them with the Band cells.
Granulocytes generating process occurs in the bone marrow. Granulocytes can be stored in the bone marrow until they are needed, then they are released to the blood stream to circulate for 12 hours in average. After circulating with the blood, Granulocytes live in the tissues for 2 to 3 days.
– Agranulocytes are the second kind of Leukocytes. ِAgranulocytes are given this name because they were believed to not have granules in their cytoplasm. It is discovered later that Agranulocytes have very small granules. The nucleus of arganulocytes look more like one block, which gave them the name Mononuclear Leukocytes.
There are 2 types of Mononuclear Leukocytes:
- Monocytes: are also formed in the bone marrow from the Myeloblast stem cell by differentiating it into Monoblast. And the monoblast is differentiated into Monocytes which leaves the boon marrow shortly to the blood stream. Monocytes can circulate with the blood for 36 hours, then they are matured in the tissues into Macrophges that can live in the tissues for months or years. Macrophages are larger in size than Monocytes and they can attack larger particles that the smaller Granulocytes cannot envelop.
- Lymphocytes: are differentiated from the Lymphoblast in the Lymphoid tissue, thymus, or spleen. The Lymphoblast is also differentiated from the Hemocytoblast (The mother of all blood cells). The Lymphocytes live for long time and they renter a cell division process to generate new Lymphocytes when needed. They don’t circulate in the blood as much as the Monocytes or Granulocytes do. But they leave the lymphatic tissue and circulate in the blood and return back to the tissue.
Those five types of white blood cells (The Neutrophils, the Eosinophils, the Basophils, the Monocytes, and the Lymphocytes) work together to attack foreign bodies that attempt to invade the human body. The increase or decrease in the cells counts for each white blood cells type can be a sign of something that is going wrong with the body or its immune system. This is why the White Blood Cells Count Test is important in diagnoses to affirm or deny the possibility of a certain disease or disorder.