<unk> s, 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<unk> s are produced and derived from multipotent cells in the bone marrow known as hematopoietic stem cells. Leukocytes are found throughout the body, including the blood and lymphatic system. All<unk> s have nuclei, which distinguishes them from the other blood cells, the anucleated red blood cells (RBCs) and platelets. The different<unk> s are usually classified by cell lineage (myeloid cells or lymphoid cells). Myeloid cells (myelocytes) include neutrophils, eosinophils, mast cells, basophils, and monocytes. Monocytes are further subdivided into dendritic cells and macrophages. Monocytes and neutrophils are phagocytic. Lymphoid cells (lymphocytes) include T cells (subdivided into helper T cells, memory T cells, cytotoxic T cells), B cells (subdivided into plasma cells and memory B cells), and natural killer cells. Historically,<unk> s were classified by their physical characteristics (granulocytes and agranulocytes), but this classification system is less frequently used now. The number of leukocytes in the blood is often an indicator of disease, and thus the<unk>  count is an important subset of the complete blood count. The normal<unk>  cell count is usually between 4 × 109/L and 1.1 × 1010/L. In the US, this is usually expressed as 4,000 to 11,000<unk> s per microliter of blood.<unk> s make up approximately 1% of the total blood volume in a healthy adult, making them substantially less numerous than the red blood cells 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. This indicates a weakened immune system.
White blood cell
Blood cell
Agranulocyte
Red blood cell
Monocyte
Podocyte
Cancer cell
Macroovalocyte