White Blood Cells

White blood cells, also called leukocytes, make up the immune system. They defend the body against infections and disease. There are many different types of specialized white blood cells, each playing a specific role in defending the body.

All white blood cells are made in the bone marrow and begin as multipotential hematopoietic stem cells. These stem cells differentiate into either lymphoid progenitor cells, which mature in specialized tissues and give rise to Natural Killer cells and T and B lymphocytes, or myeloid progenitor cells, which give rise to all other leukocytes.

The white blood cells can be split into granulocytes and agranulocytes which is determined by whether the cell possesses protein-filled granules in its cytoplasm or not.

Lymphocytes include:
- B cells
- T cells
- Natural killer cells

Myelocytes include:
- Monocytes
- Eosinophils
- Basophils
- Neutrophils

Illustration of five different types of white blood cells in line, from left to right: a Neutrophil is depicted containing a nucleus with 2 to 5 irregularly shaped lobes and granules within the cytoplasm. An Eosinophil, generally containing a bi-lobed nucleus and cytoplasmic granules. A Basophil, which contains a bi-lobed nucleus which is rarely seen owing to the high density of granules in its cytoplasm. A Monocyte, the largest white blood cell with a very big indented nucleus shaped like a kidney bean and lacking granules in the cytoplasm. A Lymphocyte, without any granules but has large nucleus which fills almost all of the cell's volume leaving only a thin rim of cytoplasm visible.

Figure 1: White blood cells (WBCs): Neutrophils, Eosinophils, Basophils, Monocytes, and Lymphocytes.