Leukocytes use the blood for transport from the bone marrow to their major sites of activity. The majority of the functions of white blood cells takes place when they leave the circulation to enter the tissues. The total number of leukocytes in the peripheral blood is normally 4.0-11.0 x 109/liter.
There are five main types of white blood cells; their names and relative proportions in circulation are as follows:
Neutrophils 40-75%
Eosinophils 5%
Basophils 0,5%
Lymphocytes 20-50%
Monocytes 1-5%
If there is a requirement for increased activity of any one cell type in the peripheral tissues, the number and proportion of that cell type rises accordingly.
Neutrophils, eosinophils and basophils are known as granulocytes because their cytoplasm contains prominent granules. Lymphocytes and monocytes are found mainly in tissues such as lymph nodes and spleen. In the tissues, monocytes transform into macrophages and basophils become mast cells.

Monocytes (monocytes/monociti/monocitos)
Monocytes are the blood and bone marrow-located precursors of macrophages found in tissues and lymphoid organs. Monocytes are large phagocytic cells (cellules phagocytaires /cellule fagocitarie/células fagocitarias). Monocytes respond chemotactically to the presence of necrotic material, invading micro-organisms and inflammation (infiammation/infiammazione/inflamación), and leave the blood to enter the tissues, where they are called macrophages (macrophages/macrofagi/macrófagos). When activated, macrophages differentiate to serve specialized roles:
- phagocytosis (ingestion and destruction of foreign material) [E]
- secretion of cytokines (cytokines/citochine/citoquinas) [E], [F], [I]
- APCs (antigen-presenting cells) [E]

Granulocytes (granulocytes/granulociti/granulocitos)
- Neutrophils (neutrophiles/neutrofili/neutrófilos) Neutrophils are the most abundant of the circulating white cells. They circulate in a resting state but, with appropriate activation, leave the blood and enter tissues where they become highly motile, phagocytic cells [E1], [E2]. Their primary function is to ingest and destroy invading micro-organisms in tissues. They play a central role in the early stages of the acute inflammatory response to tissue injury and are the major constituents of pus.
- Eosinophils (éosinophiles/eosinofili/eosinófilos)
Eosinophils are phagocytic cells, with a particular affinity for antigen-antibody complexes, but have less microbicidal activity than neutrophils.
- Basophils (basophiles/basofili/basófilos)
Basophils leave the circulation to form mast cells in the tissues. These cells are characterized by large cytoplasmic granules, which contain enzymes and substances involved in inflammation.

Dendritic Cells (cellules dendritiques/cellule dendritiche/células dendríticas)
Another cell type, addressed only recently, is the dendritic cell. Dendritic cells, which also originate in the bone marrow, function as antigen presenting cells (APC). In fact, the dendritic cells are more efficient APCs than macrophages. These cells are usually found in the structural compartment of the lymphoid organs such as the thymus, lymph nodes and spleen. However, they are also found in the bloodstream and other tissues of the body. It is believed that they capture antigens or bring them to the lymphoid organs where an immune response is initiated. Of particular interest is the recent finding that dendritic cells bind high amount of HIV, and may be a reservoir of virus that is transmitted to T helper cells during an activation event.