Leukocytosis is an increased number of leukocytes, or white blood cells (WBCs), in the blood. Neutrophils are WBCs that move from the blood into the cells to kill invading bacteria and fungi. If neutrophil levels become too high, neutrophilia results. Neutrophilia is the most common form of leukocytosis. Absolute neutrophilia refers to the increase in the total number of leukocytes in the blood as well as an increased percentage of neutrophils. This results in a neutrophil count of over 8,000.
Neutrophils are produced in the bone marrow, released into the blood, circulate briefly, and migrate into tissue spaces and onto epithelial surfaces. Injury, bacterial invasion of tissue or other causes results in the production and release of colony-stimulating factors, which increase proliferation and maturation of neutrophilic precursor cells in the bone marrow.
Other mediators of inflammation stimulate bone marrow release and promote margination and adhesion of neutrophils to vascular endothelium at the site of inflammation. The time for production of new cells from the bone marrow is 4 to 6 days. Neutrophils circulate for about 10 hours and are then compartmentalized into a circulating neutrophil pool (CNP) and a marginal neutrophil pool (MNP). Neutrophils in the CNP circulate with other blood cells and are measured in the CBC. The MNP consists of neutrophils that are intermittently adherent to endothelium, especially in small veins and capillaries.
Movement of neutrophils into tissues occurs randomly and is a one way process. Neutrophils are destroyed in the spleen, liver, and bone marrow. The number of circulating neutrophils are affected by the rate of bone marrow production and release, the rate of exchange between CNP and MNP, and the rate of migration into tissue. Neutrophilia results when the rate of marrow production and release increases, neutrophils move from the MNP into the CNP, or the tissue demand for neutrophils increases.
Among the causes of neutrophilia are infections (i.e. bacterial or fungal infections or tuberculosis), inflammatory diseases such as rheumatoid arthritis, and loss of blood cells through bleeding or hemolysis (destruction of blood cells). Other causes include cold, heat, exercise, seizures, pain, labor, surgery, panic, and rage. In addition, myeloproliferative disorders, causing proliferation of bone marrow cells, and cancer may also cause neutrophilia.
Fear, excitement, and vigorous exercise cause epinephrine release. Neutrophils demarginate from MNP into CNP, resulting in a transient (1 hour), mature neutrophilia.
Corticosteriods, whether from stress or when using drugs like prednisone, cause increased bone marrow release of mature neutrophils, movement into the CNP, and decreased tissue migration. An increase in white blood cells and neutrophilia occurs 4 to 8 hours after elevation of stress hormones or steroid use and return to normal 1 to3 days after later. Lymphopenia can occur concurrently.
Inflammation, sepsis, necrosis, and immune-mediated disease can cause increased tissue demand and increased bone marrow release of segmented and band neutrophils. Leukocytosis (15,000 to 30,000), neutrophilia with a left shift (the ratio of neutrophils to lymphocytes is high), toxic neutrophils, lymphopenia, eosinopenia, and variable monocytosis are seen often at the same time. Surgical removal or drainage of a septic focus may increase neutrophilia.