White blood cells (WBCs), also called leukocytes, are the circulating cells of the immune system. They defend the body against infections.

Leukocytes can be split into two broad classes of cells depending on their lineage. They can either be myelocytes that come from myeloid progenitor cells or lymphocytes that come from lymphoid progenitor cells. Both classes are derived originally from stem cells in the bone marrow.

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

Myelocytes include:
- Monocytes
- Eosinophils
- Basophils
- Neutrophils

Figure 1: White blood cells (WBCs)