White blood cells (WBCs), also called leukocytes or leucocytes, are the cells of the immune system that are involved in protecting the body against both infectious disease and foreign invaders. 
All white blood cells are produced and derived from a multipotent cell in the bone marrow known as a hematopoietic stem cell. Leukocytes are found throughout the body, including the blood and lymphatic system.

All white blood cells have nuclei, which distinguishes them from the other blood cells, the anucleated red blood cells (RBCs) and platelets. Types of white blood cells can be classified in standard ways. 
Two pairs of broadest categories classify them either by structure (granulocytes or agranulocytes) or by cell division lineage (myeloid cells or lymphoid cells). 
These broadest categories can be further divided into the five main types: neutrophils, eosinophils, basophils, lymphocytes, and monocytes.[2] These types are distinguished by their physical and functional characteristics.
 Monocytes and neutrophils are phagocytic. 
Further subtypes can be classified; for example, among lymphocytes, there are B cells, T cells, and NK cells.

The number of leukocytes in the blood is often an indicator of disease, and thus the WBC count is an important subset of the complete blood count. The normal white cell count is usually between 4 and 11 × 109/L. In the US this is usually expressed as 4,000–11,000 white blood cells per microliter of blood.[3] They make up approximately 1% of the total blood volume in a healthy adult,[4] making them substantially less numerous than the RBCs at 40% to 45%.
 However, this 1% of the blood makes a large difference to health, because immunity depends on it. An increase in the number of leukocytes over the upper limits is called leukocytosis. It is normal when it is part of healthy immune responses, which happen frequently.
 It is occasionally abnormal, when it is neoplastic or autoimmune in origin.
 A decrease below the lower limit is called leukopenia. It weakens the immune system.