Neutropenia

Neutrophils are the most common type of leukocytes, known colloquially as white blood cells, that normally circulate as granulocytes along the vascular endothelium in peripheral blood vessels. Neutrophils play an essential role in immune defenses because they ingest, kill, and digest invading microorganisms, including fungi and bacteria after extravasating through blood vessels, a process better known as transendothelial migration. Failure to carry out this role leads to immunodeficiency, mainly characterized by recurrent infections. Defects in neutrophil function can be quantitative, as seen in neutropenia, or qualitative, as seen in neutrophil dysfunction. The standard circulating neutrophil count should be greater than 1.5 x 10⁹/L. Neutropenia can be classified as mild, moderate, severe, and agranulocytosis. Please see StatPearls' associated reference, "Agranulocytosis," for further information on this topic. Neutropenia classification, per circulating neutrophil count, includes:

1. Mild: 1.0 to 1.5 x 10⁹/L

2. Moderate: 0.5 to 1.0 x 10⁹/L

3. Severe: less than 0.5 x 10⁹/L

4. Agranulocytosis: less than 0.2 x 10⁹/L with absence of neutrophil precursors in bone marrow

Neutropenia, or a state of low neutrophils, in itself has a wide differential that includes a primary disorder of production in the bone marrow, genetic deficiency, and cyclical deficiency. Additional causes of primary neutropenia include malignant disorders of marrow failure, such as leukopenia, or infiltrative hematologic neoplasms, such as high-grade lymphoma. Neutropenia can also be secondary to marrow extrinsic causes, including autoimmune diseases, cytotoxic chemotherapy, or suppressive illnesses or infections; even antibiotics can be toxic to neutrophils. Multifactorial neutropenia is also not uncommon, and is seen in patients who are on systemic chemotherapy and multiple broad-spectrum antibiotics., for instance.

Primary neutropenia, with decreased production and marrow hypoplasia, can be due to chronic benign neutropenia, cyclical neutropenia, or other congenital and genetic neutropenias. Acquired neutropenia can be secondary to cytotoxic drugs, leukemia, drug reactions, or infections. Congenital forms are rare and vary in severity. Some life-threatening neutropenias include leukocyte adhesion deficiency syndromes, Chediak-Higashi syndrome, Shwachman-Diamond syndrome, and chronic granulomatous disease.